Also make sure any anti-virus software didn't wake up and decide that the incoming connection is an attack.
This has nothing to do with the firewall. Even after you opened the port and said 'let connections in' some AV software will STILL think that it knows better then you.
I have had this happen at times when a definition update changes something. And then, a while later after the next update, it starts working again with no issues.

-Dave