PayPal integration failed and/or submission displaying 'Security header invalid' error message

I am getting the attached error when forms are submitted. I would like forms to only be submitted if payment has been made. 

I set up to PayPal Express wizard (via jotforms) and enabled the IPN on my PayPal business account using https://www.jotform.com/help/276-How-to-Enable-IPN-on-your-Paypal-Account 

The 'main form' is https://www.jotform.com/build/80098706084865

Generally, this error message is displayed when the API credentials are incorrect or not entered correctly. We request you to check the API keys once again or generate a new set of keys to check if that works for you. You may also check if the form is working correctly by entering the Sandbox API credentials.

If the issue still persists, please let us know. We will be happy to take a look again. Thanks!

Thanks for your reply Kiran.

Still having problems.

Question 1) Can you confirm that the API keys are the last on the PayPal attachment i.e. NVP/SOAP API Integration?

I have taken your advise and checked the keys and put the same ones in again. While your advise to generate new ones or use sandbox is good advise I am afraid to change too many things until PayPal get back to me as they do warn about deleting old keys before using new keys (I don't want to confuse the system further) - but PayPal support are much slower at responding then you guys. 

PayPal resources say this https://www.paypal.com/us/selfhelp/article/why-did-i-get-api-error-code-10008-ts1159/1   

Question 2) I don't have the option of using a payment button on my jotform. Is that the way it is supposed to work?

Question 3) Am I right in saying that when submit is pressed the user is directed to PayPal Express for payment and once this is completed the jotform will be submitted? 

Question 4) I do have a PayPal button on my website users are using until I get my fancy new jotform working. Could this be causing the problem? https://www.dogadoption.org.au/payment-options.html

Question 5) Also I previously tried to set up PayPal on a different jotform and chose sandbox but have since deleted that payment process element and the entire form. But could PayPal still think I have sandbox selected? The payment process we are discussing does not have sandbox selected. 

Grateful for any more clarity as the whole form has stalled because of the payment process. 

Can you confirm that the API keys are the last on the PayPal attachment i.e. NVP/SOAP API Integration?

I have checked your JotForm and see that the API keys seems to be correct. If you are not sure, you may confirm/generate the keys by using the link below:

https://www.paypal.com/us/cgi-bin/webscr?cmd=_get-api-signature&generic-flow=true

I don't have the option of using a payment button on my jotform. Is that the way it is supposed to work?

Am I right in saying that when submit is pressed the user is directed to PayPal Express for payment and once this is completed the jotform will be submitted? 

Yes. The submit button on the form will be used to send the information to PayPal for the payment processing.

I do have a PayPal button on my website users are using until I get my fancy new jotform working. Could this be causing the problem? https://www.dogadoption.org.au/payment-options.html

The PayPal integration on your website seems to be direct integration using PayPal button. The PayPal button on the form will be redirecting to PayPal directly to process the payments while the Submit button is used on your JotForm to redirect to PayPal for payment and store the information entered on the form as well. It shouldn't be causing any issues.

Also I previously tried to set up PayPal on a different jotform and chose sandbox but have since deleted that payment process element and the entire form. But could PayPal still think I have sandbox selected? The payment process we are discussing does not have sandbox selected. 

Please note that the Sandbox is used to test the form using test credit cards or test PayPal accounts. The sandbox mode will be enabled only if the option is selected in the Payment field settings. Otherwise, it should be processing live payments. PayPal can be processing the payments based on the option selected. 

Hope this information helps. If you need any further help, please let us know. We will be happy to help.

Thank you for taking the time to explain. Very helpful. I am learning a great deal. 

Good news - the PayPal express payment gateway is working. 

Thanks again for your patience.