@tescher This is a Nextcloud bug, please ask there. See also
java.lang.NullPointerException: Attempt to invoke virtual method 'long java.lang.Long.longValue()' on a null object reference
I used the Doodle app to add an calender entry. Since then DAVDroid fails to synchronise the account (caldav with ownCloud 8.0.3)
I could reproduce the problem, which is exactly as said as above. I have contacted Doodle about it.
Also, ical4android has been modified to be more robust in such cases. The changes will be incorporated into the next DAVdroid version.