Bug #1: Dawnguard.esm keeps crashing Creation Kit.

Bug #2, game-breaking: All vampires in the Castle of the vampire faction attack me when I load a savegame there set shortly after my defeating Harkon. Unable to diagnose, due to CK unable to load Dawnguard.esm.

If there is a solution I would love to hear it.

With the smallest amount of effort you could easily have found several threads like this: http://www.gamesas.com/topic/1400762-creation-kit-unable-to-load-the-dawnguard-esm/.
Next time, have a look around.