Thanks to T and to Ingrid the Great! for the replies. As a
workaround I just disabled backward movement in the quiz, I removed
the "Review Quiz" button, and I placed a big warning just prior to
the quiz stating that students must not try to navigate once they
enter the quiz mechanism. (Inside the quiz the back arrow is now
disabled.) Not perfect but it does work if they follow the
directions.
Before I did this workaround, I could replicate the error
every single time. The main trigger for this error was trying to
navigate backwards once you get into the quiz. Apparantly, using
the Page Up or Page Down keys were'nt the cause. It was *any*
backward navigation (or skipping a question) that caused the
freeze.