This happens when I successfully complete objectives in the Lost Legacies quests and get the pop-up telling me to go to the event UI to claim my items. Clicking the "Go to Event UI" button does not actually pull up the event UI, it just exits out of that pop-up. It seems like the event UI is about to come up, but fails. I am able to get the event UI to come up if I go there manually via the little icon on the top left of the screen, it is just an issue with the button that is included with the pop-up.
Edit By crinrict: Added Tag to Title
