PayPal Express - Security header is not valid

Support
1

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?

2

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