Traveller23, do you happen to have the latest version of “Login with Ajax” plugin installed?
I was having the same exact problem as you, and was able to determine that it’s caused by the new 3.0b version of Login with Ajax. It somehow breaks Theme My Login.
I had to go back to Login with Ajax v2.22 in order to get them both to work together again.
Also, on your page when you click “Registrieren” it pops up. What are you using for that?
Thanks!