Ok @Michael Vogel and @Hypolite Petovan - we already had the topic with "burnt" identifiers and I know when it is deleted that it is "broken". For this reason I thought that I could "save" it and then import it again (with all the keys and so on) - but apparently this is not possible - what a pity.

Because my problem is that I have two identifiers on this instance and on my mobile phone I use #Tusky as an app for Friendica.
The only strange thing is that I can integrate one identifier into Tusky, while the other identifier causes strange things. This is integrated with "@@social.fedinet.de" instead of with @zwovierzwo@social.fedinet.de" - so it is of course not possible to use this identifier in the app.
If I use this identifier in the browser, everything works fine.
As I said, another identifier works without any problems - I have also given both of them admin rights ('admin's email address'). (of course only one admin at a time)
I have also tried it on another mobile phone, but always with the same problem. For this reason, I think that something in the database has got stuck with this ID. Or that there is something wrong somewhere. The only question is where.

Deleting it and creating a new one would be the only idea I still have, but with the burnt ID it's not possible anyway.
A completely drastic step would then only be to delete the instance, create a new sub-domain and set everything up again.
The old identifiers will then be in the Fediverse for all Internet eternity and will probably be found when people search for me - but they will only be dead mailboxes.

Maybe you have another brilliant idea?