I had this problem and just now fixed it. I use Nexus Mod Manager. While going down the list of "Plugins" (installed mods), I found that some of my ".esp" files required master ".esm" files that I did not have. After disabling the ".esp" files referencing these missing mods, the game worked. I then then downloaded and installed the master ".esm" mods, enabled the slave ".esp" mods, and it still works. It seems the problem, at least for me, had to do with loading slave ".esp" files without having the master ".esm" files. If that is indeed the problem, simply download the referenced "master" mods, and that should fix it. Hope this helps.
