I got a spontaneous crash during battle as France against Scotland. There were two armies involved on each side, (reinforcements) and I have ticked the checkbox whether I want to control my reinforcements or not multiple times, and even though I left it on the option that gives me control, my reinforcements were AI-controlled anyway. Because of this, and the fact the crash happened shortly before my reinforcement army was about to engage the enemy, I'm suspecting it was a problem with AI-battle script, but who knows...