What happens if you save, exit game to desktop, reload? I've seen some glitches where that can help. What happens if you reload from a save prior to accepting the quest? Have you tried redoing the entire quest chain?
Saving, exiting, and reloading doesn't fix it.
I'd reload to a prior save, but I've completed at least a half dozen quests since then, and have gained a level or two.
It's a radiant quest, so there's no other steps to redo, really.
Hm... is there a console command to make the entire cell reset? And then delete/complete the objective?