This bug was driving me nuts. On level eight there’s a section where you have to blow up a bunch of red barrels to get to the cave section. In coop mode if you blow up the barrels the game insta crashes for everyone except the host. (This is why the game desperately needs drop in/drop out functionality).
Until they patch this bug the best thing to do is teleport past the barrels. Only one guy has to do it. At that point you’ll all be warped to the next section.
Side note: But did the barrel thing remind anyone of Borderlands?