I got an omni ant right next to a GSM short back fire and a sectorized ant. some clients can connect easy and has a stable connect, some of them get the message: not a secure network ,do you still want to connect? Even they say yes , can not connect.

Any idea?

Does anybody have antennas near to a GSM provider? Can it destruct my connect?

This has no connection with GSM antenna. Your clients receive this message because WEP is not enabled on AP. Enable WEP or configure Windows properly.

Eugene