Maintenance for the week of October 5:
• [IN PROGRESS] ESO Store and Account System for maintenance – October 8, 8:00AM EDT (12:00 UTC) – 12:00PM EDT (16:00 UTC)
This happened before One Tamriel was released, myself experiencing it at least a handful of times. Only fix was to cry on each others shoulders and leave the dungeon.
Every time this occurred there was a player in group with the quest. They wouldn't even know they had it until probing further... if that helps solve the riddle.