Quiz review locks

Who else has experienced this? It's something new to me, and I have been using SL for 15 years.

I uploaded an eLearning course to Articulate 360. At the end, there is a 10 question quiz. After passing the quiz with 80% correct or better, the user is offered the usual button to review the quiz and get the correct response to any question they answered wrong.

After pressing the review button, sometimes one of the questions will display like the image below (I've removed the content). What's important to notice is the slide has locked up, the focus on the center question pane, and the previous and next buttons are dark.

In this state, tab will select only browser controls, but selects nothing in the course player, including next and previous buttons, and escape does nothing. The entire activity is completely locked up, and the only thing available to do is close the browser without finishing, which is not what I want to force the user to do. 

What makes it more perplexing is this doesn't happen every time I use the quiz review function, and the slide that locks seems to be random. Sometimes it's the first, sometimes the second, third, sixth, seventh. I've tried to discover the cause or a pattern by trying different things, but still no clue what is happening. 

Any ideas are welcome. Thanks!