But be careful!
As far as I understand, you don’t have to register with a direct login. It is sufficient if your login (ergo: your email address) corresponds to the entry from the Forumlar (or the forum whose inbox received the invitation).
For example, if you have the address “hans.muster@bluewin.ch”, you can log in with a Google, Microsoft, GitHub or Facebook account that is also linked to the address “hans.muster@bluewin.ch”.
This error shouldn’t actually have anything to do with the AppCenter itself - it simply appears because you entered the password for one of your accounts incorrectly or don’t have an account there.
Kærar kveðjur - herzliche Grüsse
Dominik