PayPal Express: Security header is not valid.

I'm trying to test a new form to submit to Paypal as my old form has stopped working. I get the useful error message:

[PayPal Express Error]
-An error has occurred. Please contact form administrator for assistance.
[Security error]
-Security header is not valid

What is the form in question? This error usually means that there is an issue with your API credentials. Please contact PayPal support to make sure they are correct, and also make sure that you are integrating in Live mode.

Let us know if you need more help.