I also had this problem and put it down to one of the NPC mods I had. I solved it by uninstalling Face mod, then uninstall body texture mod, then uninstall CBBE. Re-install CBBE, Body texture, Face mod in that order. Then use texblend from Cailante Tools in your Skyrim data folder to fix neck seam. All should be ok after this. If not you could try opening console and then click on the NPC. Type in disable. close console. NPC will dissappear. Open console. Without clicking on anything type in enable then close console and the NPC will re-appear but with neck problem gone. I have had to do with a couple of times myself when an NPC got messed up. Hope this helps :)