Oasis

I'm afraid the issue isn't with the map itself, as I haven't encountered such problems in local network mode. Possible causes for the crashes: you and your friend have different versions of the game (3.0 and 3.1), or you or your friend have maps with mods in the Maps folder. I can only suggest installing one version of the game and clearing the aforementioned folder of modded maps (copy them somewhere so you don't lose them) and trying to create a game again. If that doesn't help, then I'm at a loss.
By the way, if all your other maps work fine in online play, then you can skip everything I wrote above. =)