Update: the problem was in dead code in my own extensions to the OCityMaps renderer which used Nominatim via geopy.geocoders at some point in the past. This code was no longer used / needed and so has been removed now.
The MapOSMatic instance currently returns “500 internal server error” only, the root cause of this is some OpenSSL error deep inside Django/Nominatim integration as far as I can tell so far.
This started after installing “Lets encrypt” and enabling SSL on another domain hosted on the same server, even though the MapOSMatic site doesn’t even use https yet.
So I’m not sure yet whether this is due to Apache configuration changes, or due to additional Pyhton packages that got installed during the process …