im just posting a solution that worked for me in the case of the DCC Resume bug that was annoying some people from long ago...

in my case it was my router messing things up with some kind of NAT that could change the port sent to User2 by the DCC SEND command

i couldnt fix it, but i found a quite easy work-around (not obvious, though) please anyone that has been affected by it, test and post comments,
thanks