JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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I opened your form and did a test submission. It was successful, it redirected me to the Thank You page after submitting the form. Here's a screenshot:
If it is still the same, please provide the URL of the webpage where the form is embedded so that we can further investigate.
Hope that helps. Thank you.