The answer
After creating an account, you receive a verification message. Follow its link and confirm your address on the page. Opening the link alone is insufficient if the page still asks you to confirm.
If a message is missing, check the address spelling and spam folder. Request a new link from the verification page if needed and use the newest message.
What to do
Check your inbox and spam folder for the atdomain.at message.
Open the latest verification link and press the confirmation button.
Then sign in again and check the account status.