Recently we are experiencing the same problem. It's not predictable when the error will occur: sometimes everything works as expected, sometimes the server starts discarding the UDP packages.
10 Aug 2012 | 21:53:19,624 | WARN | SocketReader | v2.protocol.SFSIoHandler | | Discard UDP packet from 184.71.175.150:59908, reason: Sender UDP Port doesn't match current session port: 59908 != 11827
Nothing fancy is going on: Player connects to the server, joins a zone, joins a room, starts playing. And the connection does not get lost on the client...
EDIT: Apparently this occurs as a result of a time-out. If the client doesn't send UDP-packages for 30-60 seconds, his session port changes somehow.