So I'm doing the darkbrotherhood quest after to kill an empire and when I enter the night mothers coffin the game crashes, stopping me from finishing the questline. so far reloading older saves, clearing the cashe, deleteing and reinstaling skyrim and the patch have not worked. Is there a workaround or will I have to delete Dawnguard? I don't want to lose my progress.

Had the exact problem, and it happens right after the nightmother says "sleep" right? I was only able to fix it by waiting until all the sounds ended and it was basically frozen but i had my friend ( who i was in an xbox live party with at the time) send me a message, which made the game load up the next scene and get me out. Hope this helps my friend

It has happened to me in the past, I just made sure I saved before I jumped in the coffin. Just reloaded or quit game and then reloaded - get a cup of coffee while you wait.

Yeah, I've encountered this bug before. The best thing to do is save right when you get inside the coffin when she says "sleep." Then, if your game crashes, you just reload that save file. It took me only two reloads the last time I did this quest.

It can take like 10 minutes for the scene to progress, just be patient and get a cup of coffee in the meantime.

This.
It happened to me too, but this trick works.

Also, it has nothing to do with Dawnguard... this bug has been around since Skyrim released.

The bug never happened to me until after Dawnguard though, so maybe it just makes it more likely?