Hey @wambacher, and thanks for joining the Mattermost forums!
I tried investigating your issue and does sound rather odd - it looks like everything is fine on your login page and in general everything works fine as you said, but for some reason the email verification link fails.
1) Do you have a system admin account with access to the server? Wondering if there's any messages in the server logs (Serverprotokoll), which you can access from System Konsole > Protokoll.
2) Also, wondering if you could check if there are any console errors? You can visit this page to see how to open the console on various browsers.
Please let me know if you have any trouble getting information in 1) and 2)? I'm hoping we can get this resolved for you as soon as possible.