diff --git a/calendar/inc/class.calendar_groupdav.inc.php b/calendar/inc/class.calendar_groupdav.inc.php
index 1bcc880c03..e70b8e8d00 100644
--- a/calendar/inc/class.calendar_groupdav.inc.php
+++ b/calendar/inc/class.calendar_groupdav.inc.php
@@ -734,7 +734,7 @@ class calendar_groupdav extends groupdav_handler
 					$this->groupdav->log("Both If-Match and If-Schedule-Tag-Match header given: If-Schedule-Tag-Match takes precedence for participants!");
 				}
 			}
-			//client specified a CalDAV Scheduling schedule-tag precondition
+			// check CalDAV Scheduling schedule-tag precondition
 			if ($this->use_schedule_tag && isset($_SERVER['HTTP_IF_SCHEDULE_TAG_MATCH']))
 			{
 				$schedule_tag_match = $_SERVER['HTTP_IF_SCHEDULE_TAG_MATCH'];
@@ -746,11 +746,17 @@ class calendar_groupdav extends groupdav_handler
 					if ($this->debug) error_log(__METHOD__."(,,$user) schedule_tag missmatch: given '$schedule_tag_match' != '$schedule_tag'");
 					return '412 Precondition Failed';
 				}
+			}
+			// if no edit-rights (aka no organizer), update only attendee stuff: status and alarms
+			if (!$this->check_access(EGW_ACL_EDIT, $oldEvent))
+			{
+				if (isset($oldEvent['participants'][$user]))
+				{
+					if ($this->debug) error_log(__METHOD__."(,,$user) user $user is NOT an attendee!");
+					return '403 Forbidden';
+				}
 				// update only participant status and alarms of current user
-				// fix for iCal on OS X, which uses only a schedule-tag (no etag), if event has no participants (only calendar owner)
-				// --> do regular calendar update as with matching etag (otherwise no updates possible)
-				if (!(count($oldEvent['participants']) == 1 && isset($oldEvent['participants'][$user])) &&
-					($events = $handler->icaltoegw($vCalendar)))
+				if (($events = $handler->icaltoegw($vCalendar)))
 				{
 					$modified = 0;
 					foreach($events as $n => $event)