I'll cut right to the chase.
I've searched and searched, and I haven't seen this particular issue. I've followed the directions from the sticky and still can't get this resolved.
I currently have two xbox 360's connected to the DGL-4500. Firmware is the latest. Both boxes can connect just fine, open NAT, no problems joining games, whatever. The problem arises when they try to talk to each other. I haven't changed any settings for a very long time (save for enabling Gamefuel and following the directions in the sticky) and both consoles used to work just fine together. I'm unable to join parties and games started by either console. Both have valid Xbox Live Gold accounts, paid up to two years in advance. Both boxes have stactic IP's and DNS addresses, and I've done the whole DHCP reservation thing for all devices on my network. Everything should work just fine.
Any ideas?