Is your firewall turned on? (See System Preferences, Sharing, Firewall tab.)
If so, try turning it off and see if that solves the problem. I don't know precisely how Panther client (regular Panther, not Panther Server) handles passing through data across interfaces and NAT when the firewall is enabled.
If turning the firewall off solves the problem, I suggest looking into Firewalk X2 or Intego NetBarrier as a more robust firewall that would let you specifically open up the ports you need, or leaving the firewall off altogether.