PayPal Express - Security header is not valid

Support
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It seems that when I attempt to make PayPal Card Payments online. It says the following: Security header is not valid

This is both on Live Mode and Test Mode.

Any ideas?

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There are already topics for this: Error: Security header is not valid, Paypal sandbox not working? Payment failed. Please try again. Security header is not valid, Guest link pay now button shows a blank page

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