I know this doesn't really fix the bug, but as a workaround you could try using another entrance to the underdark for now (so you can still continue playing). As far as I understand you could travel to the blighted village and enter the caves below, as you are not actively leaving the Goblin-camp there.
When in there, just jump into the big hole in the ground (with feather-falling. The couple kilometers till the Underdark might deal a bit to much fall damage otherwise : ;))