If you continuously crash when entering Fort Dawnguard for the first time, the problem may lie in your scripts. Removing the entire script folder may cause bugs with your mods but it will allow you to progress into the fort. I have narrowed the problem down to this script which seemed to be causing the crash: MannequinActivatorScript.pex
This script cause me to crash but it might have been just only for me. Try removing this script if you have it and if you don't it is another one. Just use process of elimination.