I have this exact problem in the current build of EB. While playing campaigns I will regularly get a CTD while loading the battle. I've delt with it simply by saving before every battle. It doesn't seem to care if it is a siege or pitched battle because mine CTD's equally for both. I personally thought it had something to do with the script since usually when I reloaded I forgot to start it before the battle and it worked fine. But I confess to having zero modding skills so it could be anything I guess. I would think about things that weren't changed between mods maybe? Obviously it is the same problem in both builds.