Symptom
Only one user can connect to a port-mapped service. A different user cannot connect until the base station has been restarted.
Solution
Update to AirPort software 2.0.4 or later, which allows multiple users to connect to port-mapped services.
With AirPort 2.0.2 software and base station firmware (4.0 and 4.02 only), only one external connection could be made to a mapped port, even though the application serving on that port may have been designed to handle multiple connections. If the single user disconnects, a different user cannot connect until after you restart the AirPort Base Station.