I haven't encountered this bug, but whenever I encounter a sunken treasure chest in a dungeon-- such as in Blessed Crucible in the room just before the final boss-- I move well away from the area, reloadui, and when I go back to the area it's fixed. Don't know if that would work here, but it's worth a shot.
I also dropped the quest, shut down the computer, the following day I returned to the dungeon, redid the quest but it was still bugged. I reloaded the UI, swapped characters, left, went back in, dropped and re-did the quest several more times, did it with another character successfully and sacrificed a chicken in my back garden.
Finally, the portal was where it was supposed to be and I completed the quest.
'Sharp Arrows'Mr.Garlic
Hidden by darkness, a shadow in the night,A sped arrow dissecting the gloom,Finding it's target, such delight.