I've checked all the other games and there are no problems after changing the lettersall parameter to AÄBCDEFGHIJKLMNOÖPQRSTUÜVWXYZß in the German language file. But I found another problem in the game crossword (not the same reason => another post).
Ralf Krause, the German translator, has changed the parameter in the official German language file. Therefore there will be no problems with future updates.
Thanks again, Stefan