The trigger is simply the progress of the main quest. Actually while doing quests for Barbiron did 3 reruns of this quest. the only glitch I found is that when you rerun the quest it'll tell you "cannot be repeated not this time" in this case I just followed the main quest from time to time I kill some mobs not related to the quest till it let me start it also applies for the counter when it jams and doesn't increment. In Mereholt no glitch as soon as I finish get scroll rerun didn't try scylla yet for rerun the first one run smoothly incremented as i completed the quest.
Hope it helps
