Just a little information for those who met the bug in the Touching the Sky quest.
Just after talking to Gelebor, he will open the entrance for the inner sanctum, and you have to pass trough the "Darkfall passage" portal, but sometimes this one will not appear.
It's only a graphic bug, because if you go inside the little tower, and you walk along the wall at the opposite of the entrance, you will notice that you can interact with the "invisible" portal. I've also made a video