diff --git a/calendar/inc/class.calendar_ical.inc.php b/calendar/inc/class.calendar_ical.inc.php
index 4c7d30f61f..624f340486 100644
--- a/calendar/inc/class.calendar_ical.inc.php
+++ b/calendar/inc/class.calendar_ical.inc.php
@@ -309,7 +309,15 @@ class calendar_ical extends calendar_boupdate
 
 			if (!isset(self::$tz_cache[$event['tzid']]))
 			{
-				self::$tz_cache[$event['tzid']] = calendar_timezones::DateTimeZone($event['tzid']);
+				try {
+					self::$tz_cache[$event['tzid']] = calendar_timezones::DateTimeZone($event['tzid']);
+				}
+				catch (Exception $e) {
+					// log unknown timezones
+					if (!empty($event['tzid'])) __egw_log_exception($e);
+					// default for no timezone and unkown to user timezone
+					self::$tz_cache[$event['tzid']] = Api\DateTime::$user_timezone;
+				}
 			}
 
 			if ($this->so->isWholeDay($event)) $event['whole_day'] = true;
@@ -2488,7 +2496,7 @@ class calendar_ical extends calendar_boupdate
 			$component->getAllAttributes('DTEND'),
 			$component->getAllAttributes('DURATION')) as $attributes)
 		{
-error_log(__METHOD__."() attribute=".array2string($attributes));
+			//error_log(__METHOD__."() attribute=".array2string($attributes));
 			switch ($attributes['name'])
 			{
 				case 'DTSTART':