I didn't find this bug anywhere else when I searched for it, so I hope I am not the only one having this problem:
In the quest "Awakening" in Dawnguard, I can not get in to the cavern because there is no ground before the entry. It looks like that: http://imageshack.us/photo/my-images/826/2012080600005.jpg/
I can not finish the quest, wich is very disappointing since I spent money on it. So every help is appreciated. I also can not use the crossbow on the third-person-view. And If there is an animation for the instakill with the crossbow, you just see him stand there and I can not move until I switch weapons.