The levels (rooms) that lead into the crash can't be reached right from the start of the game. Also I had to find out that it only occurs when both rooms are involved.

I currently have a savegame where one playable character stands in one of those rooms and another character stands in the other room. Once I switch the players the crash occurs after the level_load with the first wait(1).

I could try to place the characters directly into those rooms at the start of the game, but I don't know if it helps. If there's some kind of memory leak / overflow it might not happen then.