What does Security header is not valid mean?

I would suggest contacting PayPal support to see what is the exact issue. This I believe that this is an error caused by some settings in your PayPal merchant account.

Also if you can give us a link to your jotform in the meantime we could take a look at it to see if there are any issues with its settings.

Also, if you can direct us step-by-step what you are doing to get the error.