I also hit a bug with that quest playing as a DawnGuard. The Moth priest starts reading the scroll (stage 100) and then enters a 'this person is Busy' State... Your quest objective is to talk to the moth priest - who is now too busy to talk! Had to progress the quest with "SetStage dlc1vq03Hunter 200" from console to move on with the DLC so just posting in case anyone else is having the same issue while playing on the DawnGuard side.