Sadly i ran into another cause of this today.Some units, dont die upon reaching zero health.And when they dont die in combat,they cause the game to get stuck.
Situation was roughly this.
I engaged a stack in combat. We entered the combat screen and they moved their units.I then noticed that 2 of the four units were at zero health,but were still alive and moving.I had not attacked them yet at this point,they had just completed their opening moves.I attacked those two units, and they died.I then killed the 3rd foe,and attacked the fourth.
The fourth reached zero health,but didnt die.Instead it stood there in "waiting for order" mode, and the combat would not progress,and would not end.
I reloaded the save before this happened,and none of the units were at zero health at the beginning of combat,and they all died upon reaching zero health.I dont know why the difference.