Thanks for the update and great that you found the problem!
Syncing tasks does not work
I tried it with OpenTasks ans some other apps, but syncing tasks from a CalDAV server does not work with DAVdroid. For OpenTasks not even the sync account is recognised…
Any idea if this is a bug or how this can be resolved?
At the moment, OpenTasks is the only tasks app supported by DAVdroid.
Did you have a look at https://davdroid.bitfire.at/configuration/?
Installing DAVdroid after OpenTasks did solve the problem. But this sounds like a bug for me (in whatever of both applications) - what happens when I install updates in wrong order?
It’s an Android bug which has been fixed with Android 6 (this is why your Android version is important in support requests), and on https://davdroid.bitfire.at/configuration/ it’s recommended that you install OpenTasks before installing DAVdroid (first line).
The order of updates does not matter.
Happy to read that it now works for you!