This is an issue I had, and several other players lately have had.
Go into your NWN directory into the nwnplayer.ini file, go down to where it says Port 5120, and change the number to 5130.
Restart the application, and give it a shot!
<3
This keeps coming up for some folks in IRC and I'm tired of having to search for it over and over heh.
Could a friendly DM sticky this by chance?