The message is now showing up in the quiz naviation block. I have never seen this warning before. I am using 2.4+, but have been using it since December. Today while students were taking a quiz, it popped up every time they would click on a question number in the quiz navigation AND when they clicked the next button. The message actually flashed so fast I had to do a screen print to read it.
I can't seem to find any reference to this warning.