I have been having the same issue although it is not necessarily always associated with turning in a quest. I will arbitrarily be unable to interact with NPCs. Different continents, different characters. I am usually able to utilize my crafting supplies, can cook, enchant, or create something. I will be unable to log out at that time. In addition, when using the flight master I will be kicked off the flight with an error message transaction aborted: instance not found. Again, different areas, different flight paths.