Paypal Troubleshooting

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Hello, I am hoping someone will be able to help me out. I have set up Paypal payment as per the instructions on the OFN User Guide but when I try to do a payment to try it out I get an Error message across the top saying : “PayPal failed. Security header is not valid”.

I have it set to server: live, solution: Sole, Landing Page Billing.

I’ve tried the other options the user guide had as well but no luck.

Has anyone come across the same problem and come up with a solution?