t-online caldav error message after when creating a new appointment

Hi, I have a t-online.de account which hosts my calendar (access via CALDAV). I can sync the calender, and everything works fine. But when I create a new appointment in eM Client, I get an error message “Der Server antwortete mit dem Fehler: .” Then I delete the new appointment as suggested by the error dialog, synchronize again, and whohooo the new (deleted) appointment is there again (synced back from server). When I do not delete but ignore, there are two identical appointments.

Here is a snippet from the log file:

15:48:20.869|015| C: PutCommand https://spica.t-online.de/spica-calendar/caldav/principals/…ics
15:48:20.869|015| BEGIN:VCALENDAR
15:48:20.869|015| VERSION:2.0
15:48:20.869|015| PRODID Besorgt/MailClient.VObject/7.2.34062.0
15:48:20.869|015| BEGIN:VEVENT
15:48:20.869|015| UID:8a0f765d-a33b-4c26-8bbd-20911207d465
15:48:20.870|015| DTSTART;VALUE=DATE:20190526
15:48:20.870|015| DTEND;VALUE=DATE:20190527
15:48:20.870|015| TRANSP:OPAQUE
15:48:20.870|015| LAST-MODIFIED:20190508T134820Z
15:48:20.870|015| DTSTAMP:20190508T134820Z
15:48:20.870|015| CREATED:20190508T134820Z
15:48:20.870|015| SUMMARY:Test
15:48:20.870|015| CLASS:PUBLIC
15:48:20.870|015| BEGIN:VALARM
15:48:20.870|015| ACTION:DISPLAY
15:48:20.870|015| DESCRIPTION:Alarm
15:48:20.870|015| X-MOZ-LASTACK:99991231T235859Z
15:48:20.870|015| ACKNOWLEDGED:99991231T235859Z
15:48:20.870|015| TRIGGER;RELATED=START:-PT30M
15:48:20.870|015| X-WR-ALARMUID:66e25b5a-f170-407b-a073-bdbce9abaab8
15:48:20.870|015| END:VALARM
15:48:20.870|015| END:VEVENT
15:48:20.870|015| END:VCALENDAR
15:48:21.115|015| S: PUT https://spica.t-online.de/spica-calendar/caldav/principals/…ics
15:48:21.115|015| InternalServerError
15:48:21.116|015| Response, stream length:
-1, 133
15:48:21.116|015| Content:

Internal Server Error (500)

"}}Seems t-online does not like the appointment in some way.
If i do the same from my iPhone or from Outlook via CalDav Synchronizer, there is no problem.

Can anyone help ?