Pay-Per-View Plugin Does Not Reveal Content

I'm using the pay-per-view plugin on the the following page (which is part of our wpmu environment): - I have gone through all the steps to get security keys from Paypal. When I click on "Pay only .25 to see this content" I get the following error message:
There was a problem connecting to PayPal to setup your purchase. Please try again.
10002 – Security error – Security header is not valid

THE ABOVE HAS BEEN FIXED. (switched sandbox to live)

When I connect to my Paypal account I'm charged for the content, then I'm returned to the page where a button says "confirm .25 USD payment to see this content" - when I click on the button, rather than revealing the content, it returns me to the start with a "pay .25 USD to see this content".

Any assistance appreciated.