I was having this problem too, and I had also narrowed it down to nUI which was confusing because I knew that all nUI did was let you move the quest tracker. So, as suggested by Xrystal, I enabled Advanced Objectives Tracking in the Blizzard Interface Options - Objectives menu, unchecked Disable Mouseover, and resized the frame until it was quite big, then rechecked Disable Mouseover. And now it works just fine.
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