Hm, this could be possible.
I had the chance to check the WLAN function in a completly different environment and yes - I found 4 wireless networks and it was able to connect to one of them.
So I suppose the device does not have a hardware issue.
Indeed my own access point is a Fritz!box 7270 in draft n mode without any problems with my other wireless devices like Notebooks.
However it sounds very strange that one problematic device prevents the N770 to see all the other networks that are in range.
Now I hope I issue can be solved a I don´t plan to give up my Fritz!box only for the N770.

Thank you,
ReverZ