diff --git a/dav/README.md b/dav/README.md
index b4e7f3ace..a82439722 100644
--- a/dav/README.md
+++ b/dav/README.md
@@ -6,12 +6,14 @@ It's still in a very early stage, so expect major bugs. Please feel free to repo
 At the moment, the calendar system supports the following features:
 - A web-based drag&drop interface for managing events
 - All-Day-Events, Multi-Day-Events, and time-based events
+- Giving the subject, a description, a location and a color for the event (the color is not available through CalDAV, though)
 - Recurrences (not the whole set of options given in the iCalendar spec, but the most important ones)
+- Notification by e-mail. Multiple notifications can be set per event
 - Multiple calendars per user
 - Access to the events using CalDAV (using iPhone, Thunderbird Lightning etc., see below)
 - Read-only access to the friendica-native events (also using CalDAV)
 - The friendica-contacts are made available using CardDAV (confirmed to work with iOS)
-- Giving the subject, a description, a location and a color for the event (the color is not available through CalDAV, though)
+- The events of a calendar can be exported as ICS file. ICS files can be imported into a calendar
 
 
 Internationalization:
@@ -33,7 +35,6 @@ In case of errors, the SQL-statement to create the tables manually are shown in
 Functuality missing: (a.k.a. "Roadmap")
 - Sharing events; all events are private at the moment, therefore this system is not a complete replacement for the friendica-native events
 - Attendees / Collaboration
-- ICS Export and Import
 
 
 Used libraries
diff --git a/dav/calendar.friendica.fnk.php b/dav/calendar.friendica.fnk.php
index b1b0b3b35..fff3447e4 100644
--- a/dav/calendar.friendica.fnk.php
+++ b/dav/calendar.friendica.fnk.php
@@ -21,8 +21,6 @@ $GLOBALS["CALDAV_PRIVATE_SYSTEM_CALENDARS"] = array(CALDAV_FRIENDICA_MINE, CALDA
 define("CARDDAV_NAMESPACE_COMMUNITYCONTACTS", 1);
 define("CARDDAV_NAMESPACE_PHONECONTACTS", 2);
 
-define("CALDAV_DB_VERSION", 2);
-
 define("CALDAV_MAX_YEAR", date("Y") + 5);
 
 /**
@@ -107,7 +105,11 @@ function dav_compat_principal2namespace($principalUri = "")
 }
 
 
-function dav_compat_currentUserPrincipal() {
+/**
+ * @return string
+ */
+function dav_compat_currentUserPrincipal()
+{
 	$a = get_app();
 	return "principals/users/" . strtolower($a->user["nickname"]);
 }
@@ -172,12 +174,36 @@ function wdcal_calendar_factory($namespace, $namespace_id, $uri, $calendar = nul
  * @return Sabre_CalDAV_Backend_Common
  * @throws Exception
  */
-function wdcal_calendar_factory_by_id($calendar_id) {
+function wdcal_calendar_factory_by_id($calendar_id)
+{
 	$calendar = Sabre_CalDAV_Backend_Common::loadCalendarById($calendar_id);
 	return wdcal_calendar_factory($calendar["namespace"], $calendar["namespace_id"], $calendar["uri"], $calendar);
 }
 
-
+/**
+ * @param int $user_id
+ * @param bool $withcheck
+ * @return array
+ */
+function wdcal_create_std_calendars_get_statements($user_id, $withcheck = true)
+{
+	$stms = array();
+	$a = get_app();
+	$uris = array(
+		'private'                 => t("Private Calendar"),
+		CALDAV_FRIENDICA_MINE     => t("Friendica Events: Mine"),
+		CALDAV_FRIENDICA_CONTACTS => t("Friendica Events: Contacts"),
+	);
+	foreach ($uris as $uri => $name) {
+		$cals = q("SELECT * FROM %s%scalendars WHERE `namespace` = %d AND `namespace_id` = %d AND `uri` = '%s'",
+			CALDAV_SQL_DB, CALDAV_SQL_PREFIX, CALDAV_NAMESPACE_PRIVATE, IntVal($user_id), dbesc($uri));
+		if (count($cals) == 0 || !$withcheck) $stms[] =
+			sprintf("INSERT INTO %s%scalendars (`namespace`, `namespace_id`, `displayname`, `timezone`, `ctag`, `uri`, `has_vevent`, `has_vtodo`)
+				VALUES (%d, %d, '%s', '%s', 1, '%s', 1, 0)",
+				CALDAV_SQL_DB, CALDAV_SQL_PREFIX, CALDAV_NAMESPACE_PRIVATE, IntVal($user_id), dbesc($name), dbesc($a->timezone), dbesc($uri));
+	}
+	return $stms;
+}
 
 /**
  */
@@ -189,19 +215,6 @@ function wdcal_create_std_calendars()
 	$privates = q("SELECT COUNT(*) num FROM %s%scalendars WHERE `namespace` = %d AND `namespace_id` = %d", CALDAV_SQL_DB, CALDAV_SQL_PREFIX, CALDAV_NAMESPACE_PRIVATE, IntVal($a->user["uid"]));
 	if ($privates[0]["num"] > 0) return;
 
-	$uris = array(
-		'private'                 => t("Private Calendar"),
-		CALDAV_FRIENDICA_MINE     => t("Friendica Events: Mine"),
-		CALDAV_FRIENDICA_CONTACTS => t("Friendica Events: Contacts"),
-	);
-	foreach ($uris as $uri => $name) {
-		$cals = q("SELECT * FROM %s%scalendars WHERE `namespace` = %d AND `namespace_id` = %d AND `uri` = '%s'",
-			CALDAV_SQL_DB, CALDAV_SQL_PREFIX, CALDAV_NAMESPACE_PRIVATE, $a->user["uid"], dbesc($uri));
-		if (count($cals) == 0) {
-			q("INSERT INTO %s%scalendars (`namespace`, `namespace_id`, `displayname`, `timezone`, `ctag`, `uri`, `has_vevent`, `has_vtodo`) VALUES (%d, %d, '%s', '%s', 1, '%s', 1, 0)",
-				CALDAV_SQL_DB, CALDAV_SQL_PREFIX, CALDAV_NAMESPACE_PRIVATE, IntVal($a->user["uid"]), dbesc($name), dbesc($a->timezone), dbesc($uri)
-			);
-		}
-	}
-
+	$stms = wdcal_create_std_calendars_get_statements($a->user["uid"]);
+	foreach ($stms as $stmt) q($stmt);
 }
diff --git a/dav/common/calendar.fnk.php b/dav/common/calendar.fnk.php
index 01af64fb2..b03ed7bee 100644
--- a/dav/common/calendar.fnk.php
+++ b/dav/common/calendar.fnk.php
@@ -7,6 +7,8 @@ define("DAV_DISPLAYNAME", "{DAV:}displayname");
 define("DAV_CALENDARCOLOR", "{http://apple.com/ns/ical/}calendar-color");
 
 
+class DAVVersionMismatchException extends Exception {}
+
 
 class vcard_source_data_email
 {
@@ -357,8 +359,8 @@ function dav_create_empty_vevent($uid = "")
 
 
 /**
- * @param Sabre_VObject_Component_VCalendar $vObject
- * @return Sabre_VObject_Component_VEvent|null
+ * @param Sabre_VObject_Component_VEvent $vObject
+ * @return Sabre_VObject_Component|null
  */
 function dav_get_eventComponent(&$vObject)
 {
diff --git a/dav/common/dav_caldav_backend_private.inc.php b/dav/common/dav_caldav_backend_private.inc.php
index 86da0270e..6ca24c28e 100644
--- a/dav/common/dav_caldav_backend_private.inc.php
+++ b/dav/common/dav_caldav_backend_private.inc.php
@@ -152,6 +152,7 @@ class Sabre_CalDAV_Backend_Private extends Sabre_CalDAV_Backend_Common
 	 * common one is '{DAV:}displayname'.
 	 *
 	 * @param string $principalUri
+	 * @throws DAVVersionMismatchException
 	 * @return array
 	 */
 	public function getCalendarsForUser($principalUri)
@@ -162,6 +163,7 @@ class Sabre_CalDAV_Backend_Private extends Sabre_CalDAV_Backend_Common
 		$cals = q("SELECT * FROM %s%scalendars WHERE `namespace` = %d AND `namespace_id` = %d", CALDAV_SQL_DB, CALDAV_SQL_PREFIX, $this->getNamespace(), IntVal($n["namespace_id"]));
 		$ret  = array();
 		foreach ($cals as $cal) {
+			if (!isset($cal["uri"])) throw new DAVVersionMismatchException();
 			if (in_array($cal["uri"], $GLOBALS["CALDAV_PRIVATE_SYSTEM_CALENDARS"])) continue;
 
 			$dat = array(
diff --git a/dav/database-init.inc.php b/dav/database-init.inc.php
index 6372bf442..8cdd9b9b1 100644
--- a/dav/database-init.inc.php
+++ b/dav/database-init.inc.php
@@ -9,103 +9,71 @@ function dav_get_update_statements($from_version)
 {
 	$stms = array();
 
-	if ($from_version <= 0) {
-		$stms[] = "ALTER TABLE `dav_calendars` ADD `uri` VARCHAR( 50 ) NULL DEFAULT NULL AFTER `description` , ADD `has_vevent` TINYINT NOT NULL DEFAULT '1' AFTER `uri` , ADD `has_vtodo` TINYINT NOT NULL DEFAULT '1' AFTER `has_vevent`";
+	if ($from_version == 1) {
+		$stms[] = "ALTER TABLE `dav_calendarobjects`
+			ADD `calendar_id` INT NOT NULL AFTER `namespace_id` ,
+			ADD `user_temp` INT NOT NULL AFTER `calendar_id` ";
+		$stms[] = "ALTER TABLE `dav_calendarobjects`
+			ADD `componentType` ENUM( 'VEVENT', 'VTODO' ) NOT NULL DEFAULT 'VEVENT' AFTER `lastmodified` ,
+			ADD `firstOccurence` TIMESTAMP NOT NULL DEFAULT '0000-00-00 00:00:00' AFTER `componentType` ,
+			ADD `lastOccurence` TIMESTAMP NOT NULL DEFAULT '0000-00-00 00:00:00' AFTER `firstOccurence`";
+		$stms[] = "UPDATE dav_calendarobjects a JOIN dav_calendars b ON a.namespace = b.namespace AND a.namespace_id = b.namespace_id SET a.user_temp = b.uid";
+		$stms[] = "DROP TABLE IF EXISTS
+			`dav_addressbooks_community` ,
+			`dav_addressbooks_phone` ,
+			`dav_cache_synchronized` ,
+			`dav_caldav_log` ,
+			`dav_calendars` ,
+			`dav_cal_virtual_object_cache` ,
+			`dav_cards` ,
+			`dav_jqcalendar` ,
+			`dav_locks` ,
+			`dav_notifications` ;";
 
-		$stms[] = "UPDATE `dav_calendars` SET `uri` = 'private' WHERE `namespace` = 1";
-		$stms[] = "UPDATE `dav_calendars` SET `uri` = 'friendica-mine' WHERE `namespace` = 2 AND `namespace_id` = 1";
-		$stms[] = "UPDATE `dav_calendars` SET `uri` = 'friendica-contacts' WHERE `namespace` = 2 AND `namespace_id` = 2";
-		$stms[] = "ALTER TABLE `dav_calendars` DROP PRIMARY KEY ";
-		$stms[] = "ALTER TABLE `dav_calendars` ADD `id` INT NOT NULL AUTO_INCREMENT PRIMARY KEY FIRST ";
+		$stms = array_merge($stms, dav_get_create_statements(array("dav_calendarobjects")));
 
-		$stms[] = "ALTER TABLE `dav_calendarobjects` ADD `calendar_id` INT NOT NULL AFTER `id` ";
-		$stms[] = "UPDATE `dav_calendarobjects` a JOIN `dav_calendars` b ON a.`namespace` = b.`namespace` AND a.`namespace_id` = b.`namespace_id` SET a.`calendar_id` = b.`id`";
-		$stms[] = "ALTER TABLE `dav_calendarobjects` DROP `namespace` , DROP `namespace_id` ;";
-		$stms[] = "ALTER TABLE `dav_calendarobjects` ADD INDEX ( `calendar_id` ) ";
-		$stms[] = "ALTER TABLE `dav_calendarobjects` ADD `componentType` ENUM( 'VEVENT', 'VTODO' ) NOT NULL DEFAULT 'VEVENT' AFTER `calendardata` ,
-		 	ADD `firstOccurence` TIMESTAMP NOT NULL AFTER `lastmodified` ,
-			ADD `lastOccurence` TIMESTAMP NOT NULL AFTER `firstOccurence`";
+		$user_ids = q("SELECT DISTINCT `uid` FROM %s%scalendars", CALDAV_SQL_DB, CALDAV_SQL_PREFIX);
+		foreach ($user_ids as $user) $stms = array_merge($stms, wdcal_create_std_calendars_get_statements($user["uid"], false));
 
-		$stms[] = "DROP TABLE `dav_jqcalendar`";
-		$stms[] = "CREATE TABLE IF NOT EXISTS `dav_jqcalendar` (
-  			`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
-  			`ical_recurr_uri` varchar(100) DEFAULT NULL,
-  			`calendar_id` int(10) unsigned NOT NULL,
-  			`calendarobject_id` int(10) unsigned NOT NULL,
-  			`Subject` varchar(1000) NOT NULL,
-  			`StartTime` timestamp NULL DEFAULT NULL,
-  			`EndTime` timestamp NULL DEFAULT NULL,
-  			`IsEditable` tinyint(3) unsigned NOT NULL,
-  			`IsAllDayEvent` tinyint(4) NOT NULL,
-  			`IsRecurring` tinyint(4) NOT NULL,
-  			`Color` CHAR(6) DEFAULT NULL,
-  			PRIMARY KEY (`id`),
-  			KEY `calendarByStart` (`calendar_id`,`StartTime`),
-  			KEY `calendarobject_id` (`calendarobject_id`,`ical_recurr_uri`)
-			) DEFAULT CHARSET=utf8 ";
+		$stms[] = "UPDATE dav_calendarobjects a JOIN dav_calendars b
+			ON b.`namespace` = " . CALDAV_NAMESPACE_PRIVATE . " AND a.`user_temp` = b.`namespace_id` AND b.`uri` = 'private'
+			SET a.`calendar_id` = b.`id`";
 
+		$stms[] = "ALTER TABLE `dav_calendarobjects` DROP `namespace`, DROP `namespace_id`, DROP `user_temp`";
 
-		$stms[] = "ALTER TABLE `dav_notifications` ADD `calendar_id` INT NOT NULL AFTER `ical_recurr_uri` ";
-		$stms[] = "ALTER TABLE `dav_notifications` DROP INDEX `ical_uri` , ADD INDEX `ical_uri` ( `calendar_id` , `ical_uri` , `ical_recurr_uri` ) ";
-		$stms[] = "TRUNCATE TABLE `dav_notifications`";
-		$stms[] = "ALTER TABLE `dav_notifications` DROP `namespace` , DROP `namespace_id`";
-
-		$stms[] = "TRUNCATE TABLE `dav_cal_virtual_object_cache`";
-		$stme[] = "ALTER TABLE `dav_cal_virtual_object_cache` ADD `calendar_id` INT UNSIGNED NOT NULL AFTER `id` ";
-		$stms[] = "ALTER TABLE `dav_cal_virtual_object_cache` DROP INDEX `ref_type` , ADD INDEX `ref_type` ( `calendar_id` , `data_end` )  ";
-		$stms[] = "ALTER TABLE `dav_cal_virtual_object_cache` DROP `uid`, DROP `namespace` , DROP `namespace_id` ";
-
-		$stms[] = "CREATE TABLE `friendica`.`dav_cal_virtual_object_sync` (
-			`calendar_id` INT UNSIGNED NOT NULL ,
-			`date` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ,
-			PRIMARY KEY ( `calendar_id` )
-			) ENGINE=MyISAM DEFAULT CHARSET=utf8";
-
-		$stms[] = "DROP TABLE `dav_cache_synchronized` ";
-
-		$stms[] = "UPDATE `dav_calendars` SET `namespace` = 1, `namespace_id` = `uid`"; // last
-		$stms[] = "ALTER TABLE `dav_calendars` DROP `uid`";
 	}
 
 	return $stms;
 }
 
 /**
+ * @param array $except
  * @return array
  */
-function dav_get_create_statements()
+function dav_get_create_statements($except = array())
 {
 	$arr = array();
 
-	$arr[] = "CREATE TABLE IF NOT EXISTS `dav_addressbooks_community` (
-		`uid` int(11) NOT NULL,
+	if (!in_array("dav_addressbooks_community", $except)) $arr[] = "CREATE TABLE IF NOT EXISTS `dav_addressbooks_community` (
+  `uid` int(11) NOT NULL,
   `ctag` int(11) unsigned NOT NULL DEFAULT '1',
   PRIMARY KEY (`uid`)
-) ENGINE=MyISAM  DEFAULT CHARSET=utf8";
+) ENGINE=MyISAM DEFAULT CHARSET=utf8";
 
-	$arr[] = "CREATE TABLE IF NOT EXISTS `dav_addressbooks_phone` (
-		`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
+	if (!in_array("dav_addressbooks_phone", $except)) $arr[] = "CREATE TABLE IF NOT EXISTS `dav_addressbooks_phone` (
+  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
   `uid` int(11) NOT NULL,
-  `principaluri` varchar(100) COLLATE utf8_unicode_ci DEFAULT NULL,
-  `displayname` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
-  `uri` varchar(100) COLLATE utf8_unicode_ci DEFAULT NULL,
-  `description` text COLLATE utf8_unicode_ci,
+  `principaluri` varchar(100) CHARACTER SET utf8 COLLATE utf8_unicode_ci DEFAULT NULL,
+  `displayname` varchar(255) CHARACTER SET utf8 COLLATE utf8_unicode_ci DEFAULT NULL,
+  `uri` varchar(100) CHARACTER SET utf8 COLLATE utf8_unicode_ci DEFAULT NULL,
+  `description` text CHARACTER SET utf8 COLLATE utf8_unicode_ci,
   `ctag` int(11) unsigned NOT NULL DEFAULT '1',
   PRIMARY KEY (`id`),
   UNIQUE KEY `principaluri` (`principaluri`,`uri`)
 ) ENGINE=MyISAM  DEFAULT CHARSET=utf8";
 
-	$arr[] = "CREATE TABLE IF NOT EXISTS `dav_cache_synchronized` (
-		`uid` mediumint(8) unsigned NOT NULL,
-  `namespace` smallint(6) NOT NULL,
-  `namespace_id` int(11) NOT NULL,
-  `date` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
-  PRIMARY KEY (`uid`,`namespace`,`namespace_id`),
-  KEY `namespace` (`namespace`,`namespace_id`)
-) ENGINE=MyISAM DEFAULT CHARSET=utf8";
-
-	$arr[] = "CREATE TABLE IF NOT EXISTS `dav_caldav_log` (
-		`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
+	if (!in_array("dav_caldav_log", $except)) $arr[] = "CREATE TABLE IF NOT EXISTS `dav_caldav_log` (
+  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
   `uid` mediumint(9) NOT NULL,
   `ip` varchar(15) NOT NULL,
   `user_agent` varchar(100) NOT NULL,
@@ -116,127 +84,111 @@ function dav_get_create_statements()
   KEY `mitglied` (`uid`)
 ) ENGINE=MyISAM DEFAULT CHARSET=utf8";
 
-	$arr[] = "CREATE TABLE IF NOT EXISTS `dav_calendarobjects` (
-		`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
-  `namespace` mediumint(9) NOT NULL,
-  `namespace_id` int(10) unsigned NOT NULL,
+	if (!in_array("dav_calendarobjects", $except)) $arr[] = "CREATE TABLE IF NOT EXISTS `dav_calendarobjects` (
+  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
+  `calendar_id` int(11) NOT NULL,
   `calendardata` text,
   `uri` varchar(200) NOT NULL,
   `lastmodified` timestamp NULL DEFAULT NULL,
+  `componentType` enum('VEVENT','VTODO') NOT NULL DEFAULT 'VEVENT',
+  `firstOccurence` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
+  `lastOccurence` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
   `etag` varchar(15) NOT NULL,
   `size` int(10) unsigned NOT NULL,
   PRIMARY KEY (`id`),
-  UNIQUE KEY `uri` (`uri`,`namespace`,`namespace_id`),
-  KEY `namespace` (`namespace`,`namespace_id`)
+  UNIQUE KEY `uri` (`uri`),
+  KEY `calendar_id` (`calendar_id`)
 ) ENGINE=MyISAM  DEFAULT CHARSET=utf8";
 
-	$arr[] = "CREATE TABLE IF NOT EXISTS `dav_calendars` (
-		`namespace` mediumint(9) NOT NULL,
+	if (!in_array("dav_calendars", $except)) $arr[] = "CREATE TABLE IF NOT EXISTS `dav_calendars` (
+  `id` int(11) NOT NULL AUTO_INCREMENT,
+  `namespace` mediumint(9) NOT NULL,
   `namespace_id` int(10) unsigned NOT NULL,
-  `uid` mediumint(8) unsigned NOT NULL,
   `calendarorder` int(11) NOT NULL DEFAULT '1',
-  `calendarcolor` varchar(20) NOT NULL DEFAULT '#5858FF',
+  `calendarcolor` char(6) NOT NULL DEFAULT '5858FF',
   `displayname` varchar(200) NOT NULL,
   `timezone` text NOT NULL,
   `description` varchar(500) NOT NULL,
-  `uri` varchar(50) DEFAULT NULL,
+  `uri` varchar(50) NOT NULL DEFAULT '',
   `has_vevent` tinyint(4) NOT NULL DEFAULT '1',
   `has_vtodo` tinyint(4) NOT NULL DEFAULT '1',
   `ctag` int(10) unsigned NOT NULL,
-  PRIMARY KEY (`namespace`,`namespace_id`),
-  KEY `uid` (`uid`)
-) ENGINE=MyISAM DEFAULT CHARSET=utf8";
+  PRIMARY KEY (`id`),
+  UNIQUE KEY (`namespace` , `namespace_id` , `uri`),
+  KEY `uri` (`uri`)
+) ENGINE=MyISAM  DEFAULT CHARSET=utf8";
 
-	$arr[] = "CREATE TABLE IF NOT EXISTS `dav_cal_virtual_object_cache` (
-		`id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
-  `uid` mediumint(8) unsigned NOT NULL,
-  `namespace` mediumint(9) NOT NULL,
-  `namespace_id` int(11) NOT NULL DEFAULT '0',
+	if (!in_array("dav_cal_virtual_object_cache", $except)) $arr[] = "CREATE TABLE IF NOT EXISTS `dav_cal_virtual_object_cache` (
+  `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
+  `calendar_id` int(10) unsigned NOT NULL,
   `date` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
   `data_uri` char(80) NOT NULL,
-  `data_subject` varchar(1000) NOT NULL,
+  `data_summary` varchar(1000) NOT NULL,
   `data_location` varchar(1000) NOT NULL,
-  `data_description` text NOT NULL,
   `data_start` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
   `data_end` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
   `data_allday` tinyint(4) NOT NULL,
   `data_type` varchar(20) NOT NULL,
-  `ical` text NOT NULL,
-  `ical_size` int(11) NOT NULL,
-  `ical_etag` varchar(15) NOT NULL,
+  `calendardata` text NOT NULL,
+  `size` int(11) NOT NULL,
+  `etag` varchar(15) NOT NULL,
   PRIMARY KEY (`id`),
-  KEY `ref_type` (`namespace`,`namespace_id`),
-  KEY `mitglied` (`uid`,`data_end`),
-  KEY `data_uri` (`data_uri`)
-) ENGINE=MyISAM DEFAULT CHARSET=utf8";
+  KEY `data_uri` (`data_uri`),
+  KEY `ref_type` (`calendar_id`,`data_end`)
+) ENGINE=MyISAM  DEFAULT CHARSET=utf8";
 
-	$arr[] = "CREATE TABLE IF NOT EXISTS `dav_cards` (
-		`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
+	if (!in_array("dav_cal_virtual_object_sync", $except)) $arr[] = "CREATE TABLE IF NOT EXISTS `dav_cal_virtual_object_sync` (
+  `calendar_id` int(10) unsigned NOT NULL,
+  `date` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  PRIMARY KEY (`calendar_id`)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8";
+
+	if (!in_array("dav_cards", $except)) $arr[] = "CREATE TABLE IF NOT EXISTS `dav_cards` (
+  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
   `namespace` tinyint(3) unsigned NOT NULL,
   `namespace_id` int(11) unsigned NOT NULL,
   `contact` int(11) DEFAULT NULL,
-  `carddata` mediumtext COLLATE utf8_unicode_ci,
-  `uri` varchar(100) COLLATE utf8_unicode_ci DEFAULT NULL,
+  `carddata` mediumtext CHARACTER SET utf8 COLLATE utf8_unicode_ci,
+  `uri` varchar(100) CHARACTER SET utf8 COLLATE utf8_unicode_ci DEFAULT NULL,
   `lastmodified` int(11) unsigned DEFAULT NULL,
   `manually_edited` tinyint(4) NOT NULL DEFAULT '0',
   `manually_deleted` tinyint(4) NOT NULL DEFAULT '0',
-  `etag` varchar(15) COLLATE utf8_unicode_ci NOT NULL,
+  `etag` varchar(15) CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL,
   `size` int(10) unsigned NOT NULL,
   PRIMARY KEY (`id`),
   UNIQUE KEY `namespace` (`namespace`,`namespace_id`,`contact`),
   KEY `contact` (`contact`)
-) ENGINE=MyISAM  DEFAULT CHARSET=utf8";
+) ENGINE=InnoDB  DEFAULT CHARSET=utf8";
 
-	$arr[] = "CREATE TABLE IF NOT EXISTS `dav_jqcalendar` (
-		`id` int(11) NOT NULL AUTO_INCREMENT,
-  `uid` int(10) unsigned NOT NULL,
-  `ical_uri` varchar(200) NOT NULL,
-  `ical_recurr_uri` varchar(100) NOT NULL,
-  `namespace` mediumint(9) NOT NULL,
-  `namespace_id` int(11) NOT NULL,
-  `permission_edit` tinyint(4) NOT NULL DEFAULT '1',
-  `Subject` varchar(1000) DEFAULT NULL,
-  `Location` varchar(1000) DEFAULT NULL,
-  `Description` text,
+	if (!in_array("dav_jqcalendar", $except)) $arr[] = "CREATE TABLE IF NOT EXISTS `dav_jqcalendar` (
+  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
+  `ical_recurr_uri` varchar(100) DEFAULT NULL,
+  `calendar_id` int(10) unsigned NOT NULL,
+  `calendarobject_id` int(10) unsigned NOT NULL,
+  `Summary` varchar(100) NOT NULL,
   `StartTime` timestamp NULL DEFAULT NULL,
   `EndTime` timestamp NULL DEFAULT NULL,
-  `IsAllDayEvent` smallint(6) NOT NULL,
-  `Color` varchar(20) DEFAULT NULL,
-  `RecurringRule` varchar(500) DEFAULT NULL,
+  `IsEditable` tinyint(3) unsigned NOT NULL,
+  `IsAllDayEvent` tinyint(4) NOT NULL,
+  `IsRecurring` tinyint(4) NOT NULL,
+  `Color` char(6) DEFAULT NULL,
   PRIMARY KEY (`id`),
-  KEY `user` (`uid`,`StartTime`),
-  KEY `zuord_typ` (`namespace`,`namespace_id`),
-  KEY `ical_uri` (`ical_uri`,`ical_recurr_uri`)
-) ENGINE=MyISAM  DEFAULT CHARSET=utf8";
+  KEY `calendarByStart` (`calendar_id`,`StartTime`),
+  KEY `calendarobject_id` (`calendarobject_id`,`ical_recurr_uri`)
+) ENGINE=InnoDB  DEFAULT CHARSET=utf8";
 
-	$arr[] = "CREATE TABLE IF NOT EXISTS `dav_locks` (
-		`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
-  `owner` varchar(100) DEFAULT NULL,
-  `timeout` int(10) unsigned DEFAULT NULL,
-  `created` int(11) DEFAULT NULL,
-  `token` varchar(100) DEFAULT NULL,
-  `scope` tinyint(4) DEFAULT NULL,
-  `depth` tinyint(4) DEFAULT NULL,
-  `uri` text,
-  PRIMARY KEY (`id`)
-) ENGINE=MyISAM DEFAULT CHARSET=utf8";
-
-
-	$arr[] = "CREATE TABLE IF NOT EXISTS `dav_notifications` (
-		`id` int(11) NOT NULL AUTO_INCREMENT,
-  `uid` int(10) unsigned NOT NULL,
-  `ical_uri` varchar(200) NOT NULL,
-  `ical_recurr_uri` varchar(100) NOT NULL,
-  `namespace` mediumint(8) unsigned NOT NULL,
-  `namespace_id` int(10) unsigned NOT NULL,
+	if (!in_array("dav_notifications", $except)) $arr[] = "CREATE TABLE IF NOT EXISTS `dav_notifications` (
+  `id` int(11) NOT NULL AUTO_INCREMENT,
+  `ical_recurr_uri` varchar(100) DEFAULT NULL,
+  `calendar_id` int(11) NOT NULL,
+  `calendarobject_id` int(10) unsigned NOT NULL,
+  `action` enum('email','display') NOT NULL DEFAULT 'email',
   `alert_date` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
-  `rel_type` enum('second','minute','hour','day','week','month','year') NOT NULL,
-  `rel_value` mediumint(9) NOT NULL,
   `notified` tinyint(4) NOT NULL DEFAULT '0',
   PRIMARY KEY (`id`),
   KEY `notified` (`notified`,`alert_date`),
-  KEY `ical_uri` (`uid`,`ical_uri`,`ical_recurr_uri`)
-) ENGINE=MyISAM  DEFAULT CHARSET=utf8";
+  KEY `calendar_id` (`calendar_id`,`calendarobject_id`)
+) ENGINE=InnoDB  DEFAULT CHARSET=utf8";
 
 	return $arr;
 }
@@ -246,10 +198,11 @@ function dav_get_create_statements()
  */
 function dav_check_tables()
 {
-	$dbv = get_config("dav", "db_version");
-	if ($dbv == CALDAV_DB_VERSION) return 0; // Correct
-	if (is_numeric($dbv) || $dbv == "CALDAV_DB_VERSION") return 1; // Older version (update needed)
-	return -1; // Not installed
+	$x = q("DESCRIBE %s%scalendars", CALDAV_SQL_DB, CALDAV_SQL_PREFIX);
+	if (!$x) return -1;
+	if (count($x) == 9) return 1; // Version 0.1
+	if (count($x) == 12) return 0; // Correct
+	return -2; // Unknown version
 }
 
 
@@ -267,8 +220,6 @@ function dav_create_tables()
 		if ($db->error) $errors[] = $db->error;
 	}
 
-	if (count($errors) == 0) set_config("dav", "db_version", CALDAV_DB_VERSION);
-
 	return $errors;
 }
 
@@ -277,9 +228,9 @@ function dav_create_tables()
  */
 function dav_upgrade_tables()
 {
-	$dbv = get_config("dav", "db_version");
-	if ($dbv == "CALDAV_DB_VERSION") $ver = 0;
-	else $ver = IntVal($dbv);
+	$ver = dav_check_tables();
+	if (!in_array($ver, array(1) )) return array("Unknown error");
+
 	$stms   = dav_get_update_statements($ver);
 	$errors = array();
 
@@ -289,7 +240,5 @@ function dav_upgrade_tables()
 		if ($db->error) $errors[] = $db->error;
 	}
 
-	if (count($errors) == 0) set_config("dav", "db_version", CALDAV_DB_VERSION);
-
 	return $errors;
 }
\ No newline at end of file
diff --git a/dav/dav_caldav_backend_virtual_friendica.inc.php b/dav/dav_caldav_backend_virtual_friendica.inc.php
index e25dce369..4f761da1a 100644
--- a/dav/dav_caldav_backend_virtual_friendica.inc.php
+++ b/dav/dav_caldav_backend_virtual_friendica.inc.php
@@ -133,7 +133,7 @@ class Sabre_CalDAV_Backend_Friendica extends Sabre_CalDAV_Backend_Virtual
 			"is_allday"         => ($allday ? 1 : 0),
 			"is_moredays"       => (substr($start, 0, 10) != substr($finish, 0, 10)),
 			"is_recurring"      => ($row["type"] == "birthday"),
-			"color"             => "#f8f8ff",
+			"color"             => "7878ff",
 			"is_editable"       => 0,
 			"is_editable_quick" => 0,
 			"location"          => $row["location"],
diff --git a/dav/main.php b/dav/main.php
index e89320349..86c365c0a 100644
--- a/dav/main.php
+++ b/dav/main.php
@@ -118,52 +118,55 @@ function dav_content()
 	}
 
 	$x = "";
-
-	if ($a->argv[1] == "settings") {
-		return wdcal_getSettingsPage($a);
-	} elseif ($a->argv[1] == "wdcal") {
-		if (isset($a->argv[2]) && strlen($a->argv[2]) > 0) {
-			if ($a->argv[2] == "new") {
-				$o = "";
-				if (isset($_REQUEST["save"])) {
-					check_form_security_token_redirectOnErr($a->get_baseurl() . "/dav/wdcal/", "caledit");
-					$ret = wdcal_postEditPage("new", "", $a->user["uid"], $a->timezone, $a->get_baseurl() . "/dav/wdcal/");
-					if ($ret["ok"]) notice($ret["msg"]);
-					else info($ret["msg"]);
-					goaway($a->get_baseurl() . "/dav/wdcal/");
-				}
-				$o .= wdcal_getNewPage();
-				return $o;
-			} else {
-				$calendar_id = IntVal($a->argv[2]);
-				if (isset($a->argv[3]) && $a->argv[3] == "ics-export") {
-					wdcal_print_user_ics($calendar_id);
-				} elseif (isset($a->argv[3]) && $a->argv[3] == "ics-import") {
-					return wdcal_import_user_ics($calendar_id);
-				} elseif (isset($a->argv[3]) && $a->argv[3] > 0) {
-					if (isset($a->argv[4]) && $a->argv[4] == "edit") {
-						$o = "";
-						if (isset($_REQUEST["save"])) {
-							check_form_security_token_redirectOnErr($a->get_baseurl() . "/dav/wdcal/", "caledit");
-							$ret = wdcal_postEditPage($a->argv[3], $a->user["uid"], $a->timezone, $a->get_baseurl() . "/dav/wdcal/");
-							if ($ret["ok"]) notice($ret["msg"]);
-							else info($ret["msg"]);
-							goaway($a->get_baseurl() . "/dav/wdcal/");
-						}
-						$o .= wdcal_getEditPage($calendar_id, $a->argv[3]);
-						return $o;
-					} else {
-						return wdcal_getDetailPage($calendar_id, $a->argv[3]);
+	try {
+		if ($a->argv[1] == "settings") {
+			return wdcal_getSettingsPage($a);
+		} elseif ($a->argv[1] == "wdcal") {
+			if (isset($a->argv[2]) && strlen($a->argv[2]) > 0) {
+				if ($a->argv[2] == "new") {
+					$o = "";
+					if (isset($_REQUEST["save"])) {
+						check_form_security_token_redirectOnErr($a->get_baseurl() . "/dav/wdcal/", "caledit");
+						$ret = wdcal_postEditPage("new", "", $a->user["uid"], $a->timezone, $a->get_baseurl() . "/dav/wdcal/");
+						if ($ret["ok"]) notice($ret["msg"]);
+						else info($ret["msg"]);
+						goaway($a->get_baseurl() . "/dav/wdcal/");
 					}
+					$o .= wdcal_getNewPage();
+					return $o;
 				} else {
-					// @TODO Edit Calendar
+					$calendar_id = IntVal($a->argv[2]);
+					if (isset($a->argv[3]) && $a->argv[3] == "ics-export") {
+						wdcal_print_user_ics($calendar_id);
+					} elseif (isset($a->argv[3]) && $a->argv[3] == "ics-import") {
+						return wdcal_import_user_ics($calendar_id);
+					} elseif (isset($a->argv[3]) && $a->argv[3] > 0) {
+						if (isset($a->argv[4]) && $a->argv[4] == "edit") {
+							$o = "";
+							if (isset($_REQUEST["save"])) {
+								check_form_security_token_redirectOnErr($a->get_baseurl() . "/dav/wdcal/", "caledit");
+								$ret = wdcal_postEditPage($a->argv[3], $a->user["uid"], $a->timezone, $a->get_baseurl() . "/dav/wdcal/");
+								if ($ret["ok"]) notice($ret["msg"]);
+								else info($ret["msg"]);
+								goaway($a->get_baseurl() . "/dav/wdcal/");
+							}
+							$o .= wdcal_getEditPage($calendar_id, $a->argv[3]);
+							return $o;
+						} else {
+							return wdcal_getDetailPage($calendar_id, $a->argv[3]);
+						}
+					} else {
+						// @TODO Edit Calendar
+					}
 				}
+			} else {
+				$server = dav_create_server(true, true, false);
+				$cals   = dav_get_current_user_calendars($server, DAV_ACL_READ);
+				$x      = wdcal_printCalendar($cals, array(), $a->get_baseurl() . "/dav/wdcal/feed/", "week", 0, 200);
 			}
-		} else {
-			$server = dav_create_server(true, true, false);
-			$cals   = dav_get_current_user_calendars($server, DAV_ACL_READ);
-			$x      = wdcal_printCalendar($cals, array(), $a->get_baseurl() . "/dav/wdcal/feed/", "week", 0, 200);
 		}
+	} catch (DAVVersionMismatchException $e) {
+		$x = t("The current version of this plugin has not been set up correctly. Please contact the system administrator of your installation of friendica to fix this.");
 	}
 	return $x;
 }
@@ -208,7 +211,6 @@ function dav_profile_tabs_hook(&$a, &$b)
 }
 
 
-
 /**
  * @param App $a
  * @param object $b
@@ -220,9 +222,9 @@ function dav_cron(&$a, &$b)
 	$r = q("SELECT * FROM %s%snotifications WHERE `notified` = 0 AND `alert_date` <= NOW()", CALDAV_SQL_DB, CALDAV_SQL_PREFIX);
 	foreach ($r as $not) {
 		q("UPDATE %s%snotifications SET `notified` = 1 WHERE `id` = %d", CALDAV_SQL_DB, CALDAV_SQL_PREFIX, $not["id"]);
-		$event = q("SELECT * FROM %s%sjqcalendar WHERE `calendarobject_id` = %d", CALDAV_SQL_DB, CALDAV_SQL_PREFIX, $not["calendarobject_id"]);
-		$calendar = q("SELECT * FROM %s%scalendars WHERE `id` = %d", CALDAV_SQL_DB,CALDAV_SQL_PREFIX, $not["calendar_id"]);
-		$users = array();
+		$event    = q("SELECT * FROM %s%sjqcalendar WHERE `calendarobject_id` = %d", CALDAV_SQL_DB, CALDAV_SQL_PREFIX, $not["calendarobject_id"]);
+		$calendar = q("SELECT * FROM %s%scalendars WHERE `id` = %d", CALDAV_SQL_DB, CALDAV_SQL_PREFIX, $not["calendar_id"]);
+		$users    = array();
 		if (count($calendar) != 1 || count($event) == 0) continue;
 		switch ($calendar[0]["namespace"]) {
 			case CALDAV_NAMESPACE_PRIVATE:
@@ -232,20 +234,21 @@ function dav_cron(&$a, &$b)
 				break;
 		}
 		switch ($not["action"]) {
-			case "email": case "display": // @TODO implement "Display"
+			case "email":
+			case "display": // @TODO implement "Display"
 				foreach ($users as $user) {
-					$find = array( "%to%" , "%event%", "%url%");
-					$repl = array($user["username"], $event[0]["Summary"], $a->get_baseurl() . "/dav/wdcal/" . $calendar[0]["id"] . "/" . $not["calendarobject_id"] . "/");
+					$find      = array("%to%", "%event%", "%url%");
+					$repl      = array($user["username"], $event[0]["Summary"], $a->get_baseurl() . "/dav/wdcal/" . $calendar[0]["id"] . "/" . $not["calendarobject_id"] . "/");
 					$text_text = str_replace($find, $repl, "Hi %to%!\n\nThe event \"%event%\" is about to begin:\n%url%");
 					$text_html = str_replace($find, $repl, "Hi %to%!<br>\n<br>\nThe event \"%event%\" is about to begin:<br>\n<a href='" . "%url%" . "'>%url%</a>");
-					$params = array(
-						'fromName' => FRIENDICA_PLATFORM,
-						'fromEmail' => t('noreply') . '@' . $a->get_hostname(),
-						'replyTo' => t('noreply') . '@' . $a->get_hostname(),
-						'toEmail' => $user["email"],
-						'messageSubject' => t("Notification: " . $event[0]["Summary"]),
-						'htmlVersion' => $text_html,
-						'textVersion' => $text_text,
+					$params    = array(
+						'fromName'             => FRIENDICA_PLATFORM,
+						'fromEmail'            => t('noreply') . '@' . $a->get_hostname(),
+						'replyTo'              => t('noreply') . '@' . $a->get_hostname(),
+						'toEmail'              => $user["email"],
+						'messageSubject'       => t("Notification: " . $event[0]["Summary"]),
+						'htmlVersion'          => $text_html,
+						'textVersion'          => $text_text,
 						'additionalMailHeader' => "",
 					);
 					require_once('include/enotify.php');
@@ -257,8 +260,6 @@ function dav_cron(&$a, &$b)
 }
 
 
-
-
 /**
  * @param App $a
  * @param null|object $o
@@ -277,7 +278,10 @@ function dav_plugin_admin_post(&$a = null, &$o = null)
 	}
 	if (isset($_REQUEST["upgrade"])) {
 		$errs = dav_upgrade_tables();
-		if (count($errs) == 0) info(t('The database tables have been updated.') . EOL);
+		if (count($errs) == 0) {
+			renderAllCalDavEntries();
+			info(t('The database tables have been updated.') . EOL);
+		}
 		else notice(t("An error occurred during the update.") . EOL);
 	}
 }
@@ -303,18 +307,30 @@ function dav_plugin_admin(&$a, &$o)
 			$o .= t('Installed');
 			break;
 		case 1:
-			$o .= t('Upgrade needed') . "<br><br><input type='submit' name='upgrade' value='" . t('Upgrade') . "'>";
+			$o .= "<strong>" . t('Upgrade needed') . "</strong><br>" . t("Please back up all calendar data (the tables beginning with dav_*) before proceeding. While all calendar events <i>should</i> be converted to the new database structure, it's always safe to have a backup. Below, you can have a look at the database-queries that will be made when pressing the 'update'-button.") . "<br><br><input type='submit' name='upgrade' value='" . t('Upgrade') . "'>";
 			break;
 		case -1:
 			$o .= t('Not installed') . "<br><br><input type='submit' name='install' value='" . t('Install') . "'>";
 			break;
+		case -2:
+		default:
+			$o .= t('Unknown') . "<br><br>" . t("Something really went wrong. I cannot recover from this state automatically, sorry. Please go to the database backend, back up the data, and delete all tables beginning with 'dav_' manually. Afterwards, this installation routine should be able to reinitialize the tables automatically.");
+			break;
 	}
 	$o .= "<br><br>";
 
 	$o .= "<h3>" . t("Troubleshooting") . "</h3>";
 	$o .= "<h4>" . t("Manual creation of the database tables:") . "</h4>";
 	$o .= "<a href='#' onClick='\$(\"#sqlstatements\").show(); return false;'>" . t("Show SQL-statements") . "</a><blockquote style='display: none;' id='sqlstatements'><pre>";
-	$tables = dav_get_create_statements();
-	foreach ($tables as $t) $o .= escape_tags($t . "\n\n");
+	switch ($dbstatus) {
+		case 1:
+			$tables = dav_get_update_statements(1);
+			foreach ($tables as $t) $o .= escape_tags($t . ";\n\n");
+			break;
+		default:
+			$tables = dav_get_create_statements();
+			foreach ($tables as $t) $o .= escape_tags($t . ";\n\n");
+			break;
+	}
 	$o .= "</pre></blockquote>";
 }