I don't know why people are using URLs containing umlauts, but they're doing it more and more. Plus modern browsers don't shown the decrypted URL in the address bar any more. Problem: CiviCRM doesn't accept umlauts in URLs or e-mail-addresses.
Resolution 1: deactivate valdation
Resolution 2: decrypt umlauts (vaihingen-ös.de => xn--vaihingen-s-0fb.de)