So after creating a ticket i reinstalled a few times and it finally worked.
However after a few hours my ticket was resolved with the information: "We transferred your game to another server"
And it still works.
1. Login to nitrado -> don't go to web interface
2. on the left side there is a vertical menu (free translation by myself, since it's german for me) "My Services, Parent control, my account,... "
--> There should be a menu item called something like "my support requests", right below "my account"
There you should see your submitted ticket and it's status.