UPDATE
I have contacted Adrian.
Apparently there was some hacker incident, and while the data seemingly is NOT lost, “we” need a phpBB expert to repair things and migrate the whole thing to a current version of phpBB where this specific vulnerability has been removed. This probably is a lot of work because the L19 forum software hasn’t been updated in a decade or longer, and there apparently are customizations.
So, we’re currently looking for a phpBB expert.