it happened to me when i deleted the map named "temp" but maybe is not your case.
I recomend to remove from the game the downloaded meshes, and if it works, then add the meshes again carrefully to find what mesh is causing a problem.
check for duplicated particles they makes the game to crash.
And if none of this works... maybe you should delete completly the game folder and reinstall it (dont forget to back-up your screenies and stuff)