So - have you set up your forums in UTF-8 or they are still in "iso-8859-1"?
We would definitely recommend to set up them in UTF-8 for German language. Despite the German language pack is available in ISO only by now, we couldn't change it a lot, 'cause German chars are not available on our country. But you could do it easily, converting the language pack and all templates supplied in the current language pack, to UTF-8 (for example, using Notepad2 or another simple editor).
If you still would like to run the forums in ISO, then at first you could try to modify templates/email_admin_userregister_ger.txt and maybe you'd need just to change a few characters there which your server might interpret improperly.
Second, make an attempt to modify the $emailCharset option (described in the Manual - remove commenting slashes to enable it), trying to set a proper email encoding which should be passed to the "charset" header while emailing. Example: