I have seen a near similar thing, it was a router that knows about dcc and dcc resume, it altes the port numbers from the sender to the reciever, it just isnt very good at it
. hence you reciving a invalid reply, I fixed the one I ran into by setting the router to become a bridge hence it forwarded everything to the pc without any changes (might also be called DMZ dont quote me on that tho) This only works if the router isnt acting as a hub for multiple pc's of course
