If you read posts in the forums you would see this is a common occuring problem.
What happens is the factions are not loaded when you load your save. Then when you hit end of turn it crashes because there is nothing to beloning to that AI.
The easiest way to notice this bug is if you have met factions and they have disappeared from the power list. If you could actually see their territory before and when the game loads the cities are gone then it has occured.
From my experience with the bug when your save game does this it will infect any other save you try and load as well. Restart the game before loading any other save.
So if you are thinking about a long game I would make multiple saves periodically and not rely solely on the auto saves.