I did some more digging and the external views are not the real problem in my case, they seem to be just one way to trigger the crash once something is wrong. So my problem is clearly different to the one StrayWolf has.
I once managed to reproduce the error now. What happened:
I'm not flying myself, only doing CA stuff. There is a group of 2 Apaches in this mission. The number one gets taken out by enemies. Now ingame number two becomes unresponsive for me in F10, I can't assign a path or anything else. If at that moment I go to its external or assign a target for it, the mission crashes. As I said, I managed to do this exact thing twice now. I'm sure this doesn't only concern the Apaches, it happened on other vehicles as well. (Also: no textures on the Black Sharks. Hmmmm...)
In the short replay (attached) there is a discrepancy: In the end the one Apache doesn't die, they both turn south and fly away into the sunset until the mission ends. I've also attached the TacView file where you can see what really happened in game.
No idea what causes this behaviour, but I'll dig further tomorrow. Also gonna reinstall the module and see if this helps.
Thanks for your efforts. DCScrash1.rar