That sort of looping is usually ZC not finding the data it needs to proceed to the next step, so it goes back looking for it. Sometimes that's caused by country/zone name spelling mismatches. Sometimes it's caused by oddball server configurations that break sessions or break cookies. Often it's caused by addons which interfere with normal operation.

If things were "running fine all this year until this month", it begs the question: what got changed on your site/server to suddenly break it?