I get the same thing. I tried to take control over the carthies, and then I could successfully make the turn go around. But when I return control to romani, even if it's the next turn, the crash happens again. I haven't tried to do extremely much when in control over them, and I think it's something the AI wants to do that crashes it.

I would suggest to either
* Teleport an army over and smack every carthie army with auto_win
* Take control over carthies, move all characters around and cancel all recruitment/construction, then return control to romani. The carthies' AI may be broken after this move, but it could save your campaign.
* Reload an earlier save (probably the best bet), hoping that the same situation won't happen again.