Woh, that was strange, but finally I got it!
I tried both writings, "Steinwälzer" and "Steinwaelzer" and even "Steinwäzer", and after getting a message that "this user does not exist" with the last two writings, and an message that my passwort is not correct with the first writing, I tried my passwort, but used just lower-case letters (it has not just lower-case letters in it ), and THIS works...
I think that is quite unusual, I really thought it was because of the "Ä" and not because of my password...
So here I am, with my "real" name
Steinwälzer