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I have checked your JotForm account, thomasvantil, and it looks like your account doesn't have any forms in it. So I continued on by testing the integration with some of my own forms.
I have checked the SalesForce integration with my SalesForce account, and the authentication worked fine from my end:
The integration was set up successfully:
If there was an error in your SalesForce integration, please remove the existing integration (if any), and then try integrating your form again.
Please make sure you are using your correct SalesForce credentials. Make sure that there are no spaces accidentally pasted at the start of the Security Token, or its end.
If the issue persists for you by any chance, please share the link to the form you are trying to authenticate, and we'll gladly take a look. Thank you!