I tried loads of different tricks to get this quest to work, but almost gave it up as broken!
Then I had a hunch. The quest marker is almost bang on the fast travel point & I thought, maybe that was causing the bug.
So, you've got 3 other FT ponts to choose but I highly recommend Yngar's Fang to the NNW. From there, you get to do the longest helter skelter style slide in the game so far, it's a buzz! :)
That's where I went in & then I had no problem doing the quest.
Now, I could've just been lucky, but I had tried LOADS of times using all & any tricks I could think of & suggestions from the web. So I feel like I've found the fix!
Anyway, I hope so, for all the others who hit this bug.
I'd love to know & hope it helps other players.
Cheers. :)
ps. it's taken me 3 tries to get this posted! Spooky eh? :)