Yeah, sorry about that. I already tried that from the beginning (actually with :32400/web/index.html)
But now it works. Server is back and running. I have no clue, what changed.
Only error now is, that if I access the Pi server from my Windows PC, in the settings panel where I can choose the server, it now says “The server “PI-SERVER-NAME” does not allow secure connections”. And a red alert triangle at the server name. But it has a button called “Allow insecure connections”. I’m not too keen to press that one, as I’ve had problems with that in the past, making my server completely inaccessible, and requiring a complete reinstallation of the server software.
And I only ever access the PI server via my TV and its Plex app.