I have 2.1 2423 installed. I go to the multiplayer tab and choose join game and it loads the level and crashes right away to a message saying Application terminated due to exception: (in Map::lumobj) Unknown Lumobj index:0 See Details for complete message log contents.
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I'm guessing it has to do with the missing material stuff near the bottom but I can't find shit on Google. It would probably explain why the floor is black during the second that it actually loads before crashing. The single player works perfectly fine. I've played through to the middle of Shores of Hell without any problems.