I noticed that you have the white "Show Party Quests" button active on your Quest Watch window. Were you actually in a party at the time this happened? I don't really know if that will make a difference (and I'm guessing that it shouldn't), but it might be worth changing the button state if you're not in a party just to see if the error stops. I admit, it's a long shot at best, but it might be worth a try and only takes a single mouse click to change it.
Ahhhh, the vagueries of the aging mind. Wait.... What was I saying?