diff --git a/api/src/CalDAV/Handler.php b/api/src/CalDAV/Handler.php
index 767b1d5165..f7a2230546 100644
--- a/api/src/CalDAV/Handler.php
+++ b/api/src/CalDAV/Handler.php
@@ -286,9 +286,10 @@ abstract class Handler
 	 * @param int|string& $id on return self::$path_extension got removed
 	 * @param boolean& $return_no_access =false if set to true on call, instead of '403 Forbidden' the entry is returned and $return_no_access===false
 	 * @param boolean $ignore_if_match =false if true, ignore If-Match precondition
+	 * @param boolean $check_return_representation =true if true, checking for Prefer: return=representation and output it, false: no check
 	 * @return array|string entry on success, string with http-error-code on failure, null for PUT on an unknown id
 	 */
-	function _common_get_put_delete($method,&$options,&$id,&$return_no_access=false,$ignore_if_match=false)
+	function _common_get_put_delete($method,&$options,&$id,&$return_no_access=false,$ignore_if_match=false,$check_return_representation=true)
 	{
 		if (self::$path_extension) $id = basename($id,self::$path_extension);
 
@@ -327,7 +328,7 @@ abstract class Handler
 				{
 					if ($this->debug) error_log(__METHOD__."($method,path=$options[path],$id) HTTP_IF_MATCH='$_SERVER[HTTP_IF_MATCH]', etag='$etag': 412 Precondition failed".array2string($entry));
 					// honor Prefer: return=representation for 412 too (no need for client to explicitly reload)
-					$this->check_return_representation($options, $id);
+					if ($check_return_representation) $this->check_return_representation($options, $id);
 					return '412 Precondition Failed';
 				}
 			}
@@ -347,7 +348,7 @@ abstract class Handler
 				{
 					if ($this->debug) error_log(__METHOD__."($method,,$id) HTTP_IF_NONE_MATCH='$_SERVER[HTTP_IF_NONE_MATCH]', etag='$etag': 412 Precondition failed");
 					// honor Prefer: return=representation for 412 too (no need for client to explicitly reload)
-					$this->check_return_representation($options, $id);
+					if ($check_return_representation) $this->check_return_representation($options, $id);
 					return '412 Precondition Failed';
 				}
 			}
@@ -460,8 +461,8 @@ abstract class Handler
 				'neon'              => 'neon',
 				'ical4ol'			=> 'ical4ol',	// iCal4OL client
 				'evolution'         => 'evolution',	// Evolution
-				'thunderbird'       => 'thunderbird',	// SOGo connector for addressbook, no Lightning installed
-				'caldavsynchronizer'=> 'caldavsynchronizer',	// Outlook CalDAV Synchroniser (https://caldavsynchronizer.org/)
+				'thunderbird'       => 'thunderbird',	// Thunderbird 115+ (no extra Lightning) or SOGo connector for addressbook, no Lightning installed
+				'caldavsynchronizer'=> 'caldavsynchronizer',	// Outlook CalDAV Synchronizer (https://caldavsynchronizer.org/)
 				'davx5'             => 'davx5',     // DAVx5 (https://www.davx5.com/)
 			) as $pattern => $name)
 			{
diff --git a/calendar/inc/class.calendar_groupdav.inc.php b/calendar/inc/class.calendar_groupdav.inc.php
index 76e6848d8d..28cb8d1168 100644
--- a/calendar/inc/class.calendar_groupdav.inc.php
+++ b/calendar/inc/class.calendar_groupdav.inc.php
@@ -1011,14 +1011,29 @@ class calendar_groupdav extends Api\CalDAV\Handler
 		{
 			$_SERVER['HTTP_IF_SCHEDULE_TAG_MATCH'] = $_SERVER['HTTP_IF_SCHEDULE'];
 		}
+		// Thunderbird or Lightning: for "412 Precondition Failed" update only participant data, as TB offers user to overwrite server state
+		// CalDAVSynchronizer: does NOT handle 412 well and fails the complete sync, updating participant data only is the better option
+		$handle_etag_failure_with_partial_update = in_array(self::get_agent(), ['thunderbird', 'lightning', 'caldavsynchronizer']);
+
 		$return_no_access = true;	// as handled by importVCal anyway and allows it to set the status for participants
 		$oldEvent = $this->_common_get_put_delete('PUT',$options,$id,$return_no_access,
-			isset($_SERVER['HTTP_IF_SCHEDULE_TAG_MATCH']));	// dont fail with 412 Precondition Failed in that case
-		if (!is_null($oldEvent) && !is_array($oldEvent))
+			isset($_SERVER['HTTP_IF_SCHEDULE_TAG_MATCH']), !$handle_etag_failure_with_partial_update);	// dont fail with 412 Precondition Failed in that case
+
+		if ($oldEvent === '412 Precondition Failed' && $handle_etag_failure_with_partial_update)
+		{
+			$oldEvent = $this->read($id);
+			$this->caldav->log("Handing 412 Precondition Failed for Thunderbird by only updating participant data!");
+		}
+		elseif (!is_null($oldEvent) && !is_array($oldEvent))
 		{
 			if ($this->debug) error_log(__METHOD__.': '.print_r($oldEvent,true).function_backtrace());
 			return $oldEvent;
 		}
+		else
+		{
+			// set it to false, as we have no precondition failure
+			$handle_etag_failure_with_partial_update = false;
+		}
 
 		if (is_null($oldEvent) && ($user >= 0 && !$this->bo->check_perms(Acl::ADD, 0, $user) ||
 			// if we require an extra invite grant, we fail if that does not exist (bind privilege is not given in that case)
@@ -1080,7 +1095,7 @@ class calendar_groupdav extends Api\CalDAV\Handler
 
 				if ($schedule_tag_match !== $schedule_tag)
 				{
-					if ($this->debug) error_log(__METHOD__."(,,$user) schedule_tag missmatch: given '$schedule_tag_match' != '$schedule_tag'");
+					if ($this->debug) error_log(__METHOD__."(,,$user) schedule_tag mismatch: given '$schedule_tag_match' != '$schedule_tag'");
 					// honor Prefer: return=representation for 412 too (no need for client to explicitly reload)
 					$this->check_return_representation($options, $id, $user);
 					return '412 Precondition Failed';
@@ -1088,6 +1103,8 @@ class calendar_groupdav extends Api\CalDAV\Handler
 			}
 			// if no edit-rights (aka no organizer), update only attendee stuff: status and alarms
 			if (!$this->check_access(Acl::EDIT, $oldEvent) ||
+				// only update participant data, if precondition is NOT meet
+				$handle_etag_failure_with_partial_update ||
 				// we ignored Lightings If-None-Match: "*" --> do not overwrite event, just change status
 				!empty($workaround_lightning_if_none_match))
 			{