Just for interest (well i was interested) after testing it appeared that udp packets with a data length of less than 12 bytes do not make it to the receiving end. I don't know if that's the router blocking them for some reason or the isp, as i can't do any network monitoring there. But the packets do leave his computer. They just don't arrive.
LFS's initial handshake UDP packets are 4 bytes, hence the error message in LFS.
Anyway, I just wanted to know these UDP reports are indeed not LFS's problem. And that's confirmed (so i will not have any doubts in the future, because i had before).
Maybe related; I'm pretty sure the 'did not receive guest info' problems are also related to client side soft or hardware problems. I'm always keeping an eye on the occasional network bug or misconfiguration on our end, but atm i'm convinced our networking stuff is perfectly in order.