Thank you for the quick response! I read the two issues you linked and yes, it seems my problem is identical to those. Sorry for double posting and not finding those myself :/.
For me, that’s one more reason to leave Horde behind, this software is causing numerous issues in our setup.
What I still don’t really get is why everything seems to be fine as long as I only create/edit tasks from within OpenTasks. Right now I am using a collection with a task that has no value assigned to the “Priority” property (checked by downloading the ics file) and OpenTasks/DAVDroid is syncing the collection without issues.
So for everyone who can’t use the workaround described in https://forums.bitfire.at/topic/1738/task-sync-priority-without-value, having a separate “Android-only” collection seems to help, too.