It's simply a conflict with some other mod. For example, you have two mods that edit the Museum of Fine Arts object, like the Shader Mod 2 Gold and the MM2 Revisited, but the game has loaded and uses only the one of those objects, that's the one of the MM2 Revisited mod. The game can't load more than once a file with the same name, and this is why happens these conflicts with the add-ons, especially with the mods.
I've let know this to Dummiesboy, author of the MM2 Revistited, but he prefered to leave the reflections idea to the Shader Mod 2 Gold, not copying it;
here the discussion about this.
To solve the problem, add two more "!" symbols ahead the !!sm2.ar file to let use the Museum of Fine Arts object of this mod with the reflection effect.