But back to my first post: Is there any option with DAVx5 to force a (manual) sync with ignoring the ctag value? (As workaround in the meantime)
The only thing which comes into my mind is to use an old version of DAVx5, i.e. 2.6.2 or older, and then use manual sync. Is that an option?
See also https://forums.bitfire.at/topic/2225/davx5-not-syncing-tasks-with-mailbox-org. It’s basically the same problem, but it seems that mailbox.org already works now.