@Nuttso I guess the registration / Captcha loading / SMS verification just took very long, so I might have tried it multiple times. In the end Signal is blocked on all Apps and I need to wait for 6 days and my account is gone... So I will try to re-register with Mull, but yeah.
Also I was not able to log out of my phone which is strange but I guess matches the "Signal runs on one phone and one phone alone" idea. So I might have tried to log in to a device while already logged in.