Same with me. Tried with Carbonite addon the only addon enabled and had the same problem.. The quest box appears If am grouped or not. The only difference is that when I have my addons on the top of the box has "nil [29995]" <-- quest number in the top of the dialog box, where if Carbonite is the only addon active there is no such text.
Tried grouped and all other addons disabled to the same effect.. Seems like a quest dialog is not closing when you turn in a quest and get another right after..
Thrumbar
Turn out the text on top of the box if from the addon Quest Compleatist.. Disabling it removes the text and leaves an empty box.