Security header is not valid – showing during checkout process


I am getting this notice when testing the checkout process:

Security header is not valid

(see attached screenshot)

It appears when click 'checkout' – normally this would link you out to PayPal but I just get this message.

*Note – this only happens when I select 'live' in the payment settings within store settings. If I am in 'sandbox' mode, the checkout completes perfectly.

Can you help?

Thanks, Phil