Yes, I tried to disable the n mode and it made me a little bit more hopeful. Now the network scan lists all the nets in range. When I try to connect to my Fritz!box I mostly get an error message that there is a connection problem. After that message the wlan function is blocked unless I switch to offline mode and back to online mode.
I then played a little bit with the signal strength of the Fritz!box and I managed to get a internet connection for one time.
So even with disabled n mode the thing is still quite instable. I suppose there are too many other n draft router in the range. I´m not really happy with that state but I´m glad to know the reason for the problem.
BTW: The problem also exists with the OS2008HE so I´m a little bit disappointed that it has not been fixed yet.

Thank you,
ReverZ