update: The export from WoltLab is wrong; the last Event in the ical file is 16. May.
I guess this is caused by the “Sub-Categories” in Woltlab. I had to export each sub-category by its own to get it working.
Thank you for your help, this issue is solved.
Sadly, there is no error by manual sync. Is there any logfile written by the app I may provide?
Already tried to wipe data and reinstalled from F-Droid store.
Here are some random images that may give you more information.
Seriously, I have no clue what is wrong: