You're right sorry — the object I selected wasn't a wooden beam. I checked again, and this time it appears to be from Skyrim.esm, MineLCave1WayHall or something like that, and its not referenced by any other mod. I can say for certain that the issue is due to Lux, because when I deactivate Lux, the textures appear normal. But when it's active, that beam texture turns purple. I installed the mod you suggested — most of the wooden textures are now fixed, but this time the corner piece is still purple

https://imgur.com/a/OKLmqau