Yes, thank you for your support. But it is not a solution in this case.
The issue is even before getting a wireless connection to the cisco ap.
Maybe there is a kind of protection in place and the cisco ap is denying a tcp connection from the b1300 but not from my Slate / AR750S.
My AR750S is working with the same Cisco AP, at the same time with the same configuration without any problem as repeater in router mode.
Of course I tested also to connect with each GL.inet router separately to avoid possible conflicts between them as root cause.
AR750S is working anytime perfectly.
B1300 is not able to establish a connection in any case.
DNS settings like rebind protection on or off has no impact because the B1300 is not able to establish a wireless tcp connection in the first place.
It seems to be blocked from the cisco AP.
But AR750S is somehow able to connect. I read that they use different wireless device driver. For the B1300 they have to use a closed source driver because of the different chipset.