I feel like it might have something to do with my For King and Country mod that I downloaded recently. It was one of those mods that doesn't let you play vanilla unless you delete everything related to the mod, and so there might be some residual files from that mod that are there, still screwing up 1648. Either that or during my search for For King and Country-related files in steamapps/common/Medieval 2 Total War, I accidentally deleted some files that were necessary for 1648 to run. But vanilla Med2 loads just fine as of now.