My friend and I are having the same problem. We can both play quick match games, but if we try to enter a buddy list game. As far as I know we haven't been able to do this since the servers switched to xwis. I think it may be an issue with how xwis handles buddy list games.
For us we can set up the game, but once everyone has clicked "ready" and the game is about to start, it shows the bars for connecting to other players, and they don't go up at all. Then the multiplayer game exits and you are taken back to the buddy list screen (I think. I know you are still in the game). Basically it acts as though the connection failed.
We are able to play LAN games (and as I said above online quickmatch games). Any help would be appreciated.
I would also be interested if there is a mod that would allow you to direct connect to another player over the internet.