That said, if the problem started when doing the expansion, then chances are it's probably related to any mod updates to do with that expansion.
Trying to figure out what it is at random points is literally like looking for a needle in a haystack. There really isn't a simple answer, and it takes a while to get every mod correctly loaded and set up before you start to play and avoid any problems down the line. Mods and even the unofficial patch will usually have different versions for the base game and any additional expansions that change how the game loads certain files, so it can depend on a variety of factors that could get triggered at any point.
