The general cause appears to be something happening that there is no time to draw on screen and then referencing the undrawn object that should have been there. It is a general issue with the game engine, also known from New Vegas and Oldrim, that it does not have a trap for that condition but just abends. Now, with a mod collecction installed I keep getting the problem with sorcerer fort east of Dragonbridge. In Oldrim the area around Whiterun was notorious for that problem. One not unreasonable asumption is that there is a maximum for objects in a cell that is exceeded when adding too many critters to the game. In New Vegas invisible actors that should just shimmer could cause it. Can we please have a 64 bit New Vegas btw. ... please, please, please ..... PLEASE!