WordPress.com Stats plugin not working

Hi;

I’m using the “WordPress.com Stats” plugin on several blogs already and recently tried to add a new one. The domain is redirected internally to a sub-directory.
When I try to add the API key, the server responds with a “500 error”.
Url to my blog Geburtstag – Jubiläum – Jahrestag
I have another domain hosted on the same server but without redirection. WordPress.com Stats works fine here.
Url to blog Suchmaschinenoptimierung mit/für WordPress Blog

Any suggestions?
Thomas Gaida
Lizenzfreie Fotos, Sprüche und Grusskarten