Yes, I think you’re right.
I checked some other apps; all of them were synchronized on November 8th for the last time - which is exactly the date when the phone was updated to Android 6.0.1 Marshmallow. Therefore it might be a problem caused by this release!?
same calendar name, other ID?
for some reason I sometimes delete a calendar on the server and create a new one with the same name. (I don’t know any easier way to delete all entries in a calendar - do you? In Horde I could import an empty calendar (and tick the “overwrite all existing entries” box), but I’d have to create this empty file - besides that - no idea)
In davdroid nothing changes - same calendar name. However I got the impression that davdroid has some problem with the account afterwards (got sync errors or hanging sync) … Could you check that?
I don’t understand your problem. Please provide information as described in PLEASE READ BEFORE POSTING: What’s required to diagnose a problem.