From "Review in progress" to "Ready to review"
I had a similar experience. I contacted the Editor, and it turned out that one of the reviewers they contacted backed out of reviewing my paper, so the progress went back to something like "ready to review" (not those exact same words, I can't remember the exact status, but basically is a step back from being reviewed or in progress). I never would have known that if I didn't contact the Editor. A polite email to the editor won't hurt. IMHO.