in the meantime i did upgrade the recovery via heimdall and just in case also sideloaded A16 via ADB from the new recovery, but it still fails with the above error message.
Not sure whether this was already there before but now it says at the end of the message:
Reason: init_user0_failed
would there still be a chance to keep my data? –> e.g.: Would “Format cache partition” from the recovery be an option or should this not be done directly after/during an upgrade?
PS, this would belong here but that thread is locked 
Edit: was still locked when I started my first post here in this thread. – could be moved there if deemed appropriate