I know there are a lot of these, but I know how to say more than "it doesn't work"

Game runs, and I can connect to the world server just fine. I can even play by myself and it works. But when I go to menu->Multiplayer->Join game it says it's checking for game servers, and then crashes to the console with the error message:

ERROR: NET_SendPacket ERROR: WSAEINVAL to 00000000:7a7905e923c6:10983

And esrv.log reads:

--------------------------------------------------------------------------------
09/18/08 00:35:19
ERROR HTTP_GetPage unable to connect socket 2.33.186.213 WSAETIMEDOUT
--------------------------------------------------------------------------------
09/18/08 00:35:19
WARNING HTTP_ThreadSendRpc Retry HTTP_GetPage www.coopordie.co
--------------------------------------------------------------------------------
09/18/08 00:35:23
WARNING HTTP_GetPage func=GameLoop 24765 ms

I was using this with the standard quake engine at first, but now I've also tried it with nocheat and Quake2xp, and the same thing happens each time. I haven't yet tried using a VPN (hamachi or openVPN) but I'll be trying that tonight when my friend and I try again to play.

Is this a firewall/network issue or is something else going on?

Okay, using Hamachi and connecting from the command line, I can connect. Soo...I don't know what's causing it, but I can get around it.

Your bug seem very similar to the well know "WSACONNRESET" bug of classic Quake2 engine. I don't know the "nocheat" engine, but I'm surprised if Quake2xp doesn't have the fix for this.

Could you try with R1Q2 or EGL engines, theoretically this bug shouldn't occurs with them.