I found the problem at least on one machine. It was actually a very simple fix, but I have no idea why D3 is causing trouble with it. I've changed the port forwarding method, so hopefully there will be no further issues with this.
Monday is always a holiday for me.

I'll have a decent amount of work over the weekend, although I should have at least one night available.
The hoard games were a lot of fun last time. That game type actually originates from D2, so we can also play it in there. Not all D2 levels have the hoard goals though. I know Dogfight does (referring to the Parallax-made four level pack, not karx's one) and Panic does not, but I'm not too sure about any others.
Also, here is the B-side pack I was referring to, which is definitely worth playing at some point:
http://home.comcast.net/~cp5670/bside.rar