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I've tried cloning your JotForm and I was not able to reproduce the issue. As I tried to submit your JotForm, I see that the submission didn't go through and Thank you page is not displayed. Were there any recent changes made to your JotForm since it worked last time. Could you try restoring the form to the recent version that worked and see if that works?
Let us know if the issue still persists. We will be happy to look into this issue further.