Try to check your few mods. I also used few mods of mine during my play 4 times alone & with friends, with high graphic settings too, & none of us encountered crash in any levels of this campaign. & also try to check special infected skin mods if you have some of them enabled, because on certain level, there's a different tank. & special infected skin mods can also cause a crash sometimes, like I ever encountered a crash when I used a certain witch skin mod in certain custom campaigns.