I sometimes have the issue that trade windows to hand in quest items do not open anymore when they should do so during quest conversations. This issue seems to be triggered when a quest trade window is opening but I click or press some other UI input fast. The trade window then disappears and subsequently no trade windows open in any quests anymore until I restart the client. This is a minor issue once a player knows this problem can happen as restarting the client solves it. Players, especially new players, who are not aware of this bug might get stuck and permanently frustrated during quests however.