I used to get this kind of error in RTW. A campaign can get corrupted somehow and keep crashing at the same point. The only real solution was to start a new campaign, which really is not a solution at all.

I havn't seen this with M2:TW myself yet, but it happened to me a lot with RTW (no mods or any other hacks used, apart from patches). I hope this bug hasn't made it into M2:TW!