Watch no longer showing caldav calendar

Since a few days my caldav calendar (self hosted on my NAS) disappeared from my Apple watch (series 5).

I can still see all calendar events on my iPhone 11 but somehow they do not make it to the watch.

Exchange calendar and iCloud calendars are perfectly synced but not the caldav calendar.

Also on my mac I have no problems with the caldav calendar.

I've already tried to remove all my calendars, reset the data sync with watch, even unpaired the watch completely but nothing helped.

Also setting up a completly new caldav calendar (with different credentials) did not work.

I have no idea whats going on or how to fix this.