Another chime in. Id say this sounds like a port forwarding issue, but for reasons different to the two above posters.
It sounds to me like you do have port forwarding enabled and that may be the cause of your problem. You cannot forward the same port from one originating IP (the router, your external IP) to multiple destination IPs (your client PCs, internal IP).
Think of port forwarding like a pidgeon hole mailbox in a flat entrance way. You stick a note on your mailbox saying any post to go there should be put into box 10 instead. Post now goes into box 10, as requested. You now stick another note on there saying post to go to box 12. Postman sees a note saying box 10, and also box 12, only has one copy of the letter and so can only put it into one box. Maybe if he was a super expensive feature rich postman he could cope, but he isnt. He simply cant deliver one letter to two addresses.
EDIT: Actually now that I've had a cup of tea and thought about it, port forwarding should not be causing these issues. I could imagine possible problems with P2P powered patchers (which would benefit from port forwarding) but you absolutely should not need any forwarded ports to simply play a game. Maybe if you were hosting an XIV server you would, but just playing with a client not really. Then again, this is SE!
Edited, Apr 2nd 2012 4:34pm by Kordain
To endanger the soul endangers all,
when the soul is endangered it must become a Warrior.