Nah, I think you're right. Another poster found an issue with Netgear routers doing stateful packet inspection that ALSO failed. So, it sounds like mIRC had a bug, routers tried to apply some protection and also failed.
DCC doesn't work through NAT, as it creates a fresh connection to a random port. That's the entire contents of the DCC send message, the ip/port of the sender and the filename.
I'm pretty sure these Netgear routers were trying to create an automatic port forwarding rule so the DCC connection would work through NAT without any configuration.
