Another thing to note (SPOILER BELOW)
That Sanctuary will continue to freeze like that after this quest line is over and the Dark Brotherhood moves to Dawnstar. You can still go inside the "old" sanctuary, but you will suffer a 40-60 second "freeze" while it loads. I have no idea why this happens. But the reason I'm posting this (AGAIN SPOILERS FOLLOW) is that there is a Barenziah stone in the sanctuary that I did not notice until after the fire burned the place out. I ended up going back there just to see what kind of condition the place was in and if the guards are still patrolling the area and I walked into Astrid's old room, and there was the Barenziah stone.