2.0.1: Usernames with umlauts invalid?

Do all the other ‘Umlaut’-Users have the same problem or can they log in ?

That part has changed, the usernames are more restricted (to ASCII). But as far as I understood the code, the restrictions should only apply to new users, not to existing ones. You should insert an error report into trac.

http://trac.wordpress.org/

Maybe your user can use the “send password” functionality, to get his password and change it afterwards.

As I see it, the only things you can do, is
– change the user names to non umlauts or
– switch back to 2.0 (might be difficult without backup …)

There’s a third way: only change wp-include/functions-formatting.php

the function sanitize_user( $username, $strict = false)

should contain only the old line

return preg_replace('|a-z0-9 _.-|i', '', $username);

But that’s definitely a hack !

Usernames and display names are seperate, so you can still probably do whatever you want with your display name.

Having nice clean usernames is very helpful from a coding viewpoint.