CalDav-Sync via nextcloud/DavX5 (Android) stopped working -> error HTTP 500

Dear Community,

I have been using OwnCloud to sync my 5 CalDav-Calendars for years succesfully.
Syncing devices are: Two Android Phones (Samsung) using nextcloud and DAVx5 and two PCs using thunderbird.

The error
Unfortunately I’ve been having the same error for the last few weeks on one Android device. This results in not being able to sync properly.
The push message reads HTTP-Servererror 500.
The logfile says: EXCEPTION at.bitfire.dav4jvm.exception.ServiceUnavailableException: HTTP 503

What I did so far:
I updated to the latest versions of: PlayServices, Nextcloud, DavX5
I reinstalled the calendar-acces within davX5 and nextcloud.

Here is where my logic stops and where I’m kindly asking for assistance:
a) I can sync my calendars sometimes. When disabling four of my five calendars, the remaining one is more likely to sync.
b) I logged in over the webinterface of nextcloud (also here only able to login after a few attempts).
c) Some dates within the calendar are in full color, others appear in kind of a pastel tone.

Now my question is, why does the sync no longer work? What can i do to get it working properly again? It always did work until lately.

Looking forward to any hints.

Kind regards