Fixed for me! How?:
I found multiple unrelated issues that each didn’t directly have to do with Z-Push with debugging via the LOGLEVEL_WBXML loglevel.
It’s very verbose but also very helpful - seems the issue was related to the users login identifier not matching with the actual caldav principal name with one of our test users, we had other errors with other users as well, but those can be identified and troubleshooted easily with the debug logs.