Yesterday and today i have had the same issue, 3 times yesterday just once today. Using the links on top after purchasing items. When i click complete a window pops up with just 404 in the middle. When i close and go to the quest page it is just to start a new quest. I dont have the items and I honestly dont pay enough attention to see if there was a reward. Edit to add: It happened during the Minion quest. The last item i purchased was from the NPC shop. It only happened once today but I only had two quest that had items from NPC shop.