thanks a lot for your reply! That's interesting, as I did not (on purpose) load a beta version but updated from my older version via the AppStore.
However, I just solved the issue the following way:
I uninstalled the problematic Enpass 6.7.0 (876) version (via AppCleaner) and re-installed it via the AppStore. Now I have version 6.7.1 (878) (released later the day I installed 876), in which the sync-server can be successfully started.
I am not sure if it was due to the re-installation or the fact that I now have an even newer software version.
In case someone else has that issue, re-installing should probably fix it.