A service guy tried to help me. But he could not. He did not know what a LAN is at all. In the end he suggested, that I should shut down the modem, and then my LAN would work he suggested. No further questions were necessary. The YouSee supporters apparantly don't know anything about simple web techlology.
If you know a better service provider than YouSee let me know in the comments.
Unfortunately my conclusion is:
YouSee sucks big time if you want to do something "advanced". Like setting up a server om a localhost ... So basicly the service is expensive and useless.