Thanks for you clarification. I posted this to this forum, because DavDroid is the part in my environment where I see the error. The other parts (OwnCloud and Thunderbird) seemed to worked correctly from my view. But you’re right: the root cause is very likely to be in another part of my environment.
Just out of curiosity and because it worked well in the past or at leased seemed to work well:
As I set up my OwnCloud a few months ago I tested to add/modify/sync/delete contacts and calenders in every direction (Android <-> OC <-> Thunderbird). And at the time of setup everything worked without issues.
What’s the background/meaning of the ETag? Does it identify a contact, so is it like a unique ID?
Did DavDriod always check for ETag? Or was behaviour changed in the last months in order to be more conform with the RFCs?
What part creates/maintains the ETag? The client who creates/modify a contact? Or the server?
Just wanna clarify for me which part to spot next…