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I test submitted your jotform http://www.jotform.us/form/51476149126154 that have the Spreadsheet widget on it.
and it seems to be working just fine. I was able to submit the form successfully during my test submission as I was able to see the Thank You page.
Please let us know if issue still persist.
You're right. Yesterday I had multiple people try it and it all got the failure to submit when the Spreadsheet was hidden. But today I found it worked. It was strange, but thanks for checking in on it!
Thank you for taking the time letting us know all is good now.
Contact us again anytime should you need assistance on JotForm