Now I realized a new issue, which may result from the obove solution:
- two appointments for March 25 and 26, both at 7 pm changed to 8 pm in the webinterface.
- in iPad, iPhone and Outlook (all synched by CalDAV) the appointments stayed by 7pm !!
So now I have two different schedules for these appointments: In the webinterface it is 8 pm, in the synched devices it is 7 pm.
Tmezone is set to UTC +1 (Rome, Paris, Berlin, ...)
The appointments were created before installing the custom build 13.3.5556.