Can't do sieges

Every time I try to do a siege I get an 'unknown graphics error.' The only way to proceed is to automatically resolve the siege. I have tried changing graphics settings and dropping the resolution but it doesn't help. I have also tried playing with a program monitoring the GPU use and it never gets above 50% useage and does not overheat so it doesn't sound like the card is being stressed much. Anybody else having this problem?

just tried to do a siege battle and have had the same issue. which is a shame as this is supposed to be my first battle for my campaign. I have already set my resolution lower which stopped another random CTD from occurring.

Just an update. I am now in summer 157 a.d. and I am pretty much dominant playing as Romans. It has been a very hard and rewarding game which I have enjoyed very much. But I am a bit disappointed never to have seen a city! I cannot besiege or defend a siege or fight a battle which is next to a settlement involves the garrison of the settlement. All result in a crash. I wonder that noone else (apart from one) has this problem. I am using the non-steam version.