I'd recommend making chatcache 1 permanent anyway as it will improve performance on ATI cards, basically the way Toribash works with ATI cards isn't particularly elegant, and you end up using twice the VRAM that the game needs, /opt chatcache <1..5> makes it use the normal memory amount.
I'm not sure exactly how to fix this problem, you could try updating visual C++ as described in the thread, as that only effects multiplayer, and you said the problem is only in multiplayer.