During map generation. Actually, we were creating the game when the maps were in the "receiving" status. That is, they hadn't fully loaded yet.

I think these crashes can be reproduced by somehow limiting the connection speed between the games. For example, set it to 1-2 KB/s. Then, create a game and have the host close the game during the data transfer process. The second player will then experience a crash.
Or, as happened here, quickly create a game at the moment when the maps haven't fully loaded.