Just in the last two days (5.0591) I've had this problem on only one of my toons where all of the quests I'm watching (I watch everything in my quest log) gets automatically dropped whenever I do anything that changes the status of a quest, such as accepting, handing in, or abandoning a quest. Even just fulfilling one tick in a quest (i.e. going from Fish Cake: 3/5 to Fish Cake: 4/5) causes this. I've seen this both with all of my other add-ons enabled as well as with everything else besides Carbonite disabled.
Has anyone else had this problem, and if so, did you find a fix for it?