I have Blue Marble, but that was installed about a year ago so I doubt it's effect on my game.
Yeah, I seriously doubt that's the problem.
I don't know if I installed BUG as a mod.
Well, stuff installed to the CustomAssets folder is ignored by all multiplayer games. When using the EXE installer as you did, choosing "multiplayer install" installs BUG as a mod, which is what you'll need AFAIK. If you chose the "singleplayer install", BUG is in CustomAssets and shouldn't interfere with any multiplayer games.
The thing with multiplayer games, however, is that everyone must be using the same mod for it to work. If the host isn't using BUG when starting the game, I would expect a warning message and being kicked out rather than a CTD. Unfortunately, I haven't played any multiplayer BTS games yet--only testing using two instances on my machine.
Look where the BTS application is installed. It will have a Mods folder, and BUG should be installed in there as "BUG Mod 3.6".
Also, what's the difference between the EXE installer and the SVN code repository?
We do releases of the EXE every 3-4 months after more thorough testing. What's in the SVN is directly as we code it. You'll get the latest features before the EXE, but if you update frequently you might experience bugs. We're pretty good about fixing them quickly once reported, though. Check out the first post of the download thread for instructions on accessing SVN.
Finally, are you using the same BTS version as the host? I know there have been some troubles reported with the latest 3.19 patch and pitboss/PBEM games. I'm pretty sure everyone must be using the same patch.
Have you tried a normal multiplayer game with one of your friends to verify that LAN / INTERNET work as expected? It's always good to eliminate as many variables as possible.