Not sure of the cause.  If it can help you find an error log.  2 of them about 9:30 and 9:50 pm EST under this user.  I was logged in.

I created a mario-bro's faction and went without making cities.  Not sure if that has anything to do with it, but...

At the end of the turn, when I have an enemy selected to view their stats for the next turn,... and then hit the turn button, I've gotten 2 instances of the AI army's size doubling in the army view window and the turn freezes.

So, for example, I click on princopeenys army and it shows her and her one hero.

I click end turn, and her amy size doubles in the army window.  2 of her, and 2 of her hero.  2 of the 4 are whited out and the turn hangs.

Hope that helps.  Nothing is shown different visibly on the map, only in the army window.