Played around a bit more. Had this activated:
The front page url contains the language code instead of the page name or page id
If I deactivate it, the navigation will work because it will point to the slug like bla.com/wordpress/welcome and bla.de/wordpress/willkommen
which is not so nice structure.
Even then the url bla.de/wordpress/ will redirect to bla.com/wordpress/ when no slug is entered after the root dir of the wordpress install.
Guess I'll have to restructure my site to get "language per domain" working.