I guess I found 1 reason, if I upgrade my mercenary dudes to Mercenary Crossbowmen and then want to select the Crossbowmen it crashes and gives me the runtime error. I'm pretty sure it is linked to this cuz i did it a couple of time now. Yes it is clear now, just tried it again. Hope i could help
Edit: Caravan Guard seems to crash too. So people keep your fingers from the mercenary tree.