Hello, newbie here! Also german mother tongue, so thanks for being lenient with my language.
Had the same difficulties to access 1.23.4, so i followed the procedure to instal 1.25.5 manually. I also was patient to wait for the db-upgrade overnight, all as described in this thread.
Next day i opened the interface, but still couldn’t connect to the server (didn’t show up in devices section [Zugelassene Geräte ]. In a fit of overconfidence i removed 1.25.5 in the App-Center and reinstalled 1.23.4 there. Result: local.host/32400/web showed my user, but when trying to connect i got a no access to server error [Keine Berechtigung - Du hast für diesen Server keine Zugriffsberechtigung.]
So i removed 1.23.5 restarted the system and got back to 1.25.5. Same situation. Funny thing though: i could ad a new (non-admin) user, log in to the dashboard and change to the main user there. But still no Media Server in the devices section.