Let us know if deleting the LW prefs folder fixes it or not. BTW the new version is LW 4.12.11 ... easy mistake to make.
Airports have the option of port forwarding or a NAT-PMP mode that's roughly comparable to UPnP mode. If it's not working reliably in that mode then port forwarding is the way to go;
Apple Airport Extreme/Express for Mac users * (NAT-PMP)