I've had that problem at times, also. I ended up saving not far from the Sanctuary, then fast traveled the last few hundred yards. Usually it worked, sometimes, not. One other thing that worked; since I normally play in first person, I walked up looking at the ground a few feet in front of me until I got to the door. Later in the game, things seemed to be fine. Might not work for you, but most anything is worth a try....