Pay Per View Plugin Issue - user cannot complete a purchase with Paypal

The plugin and purchases were working fine at http://www.energyquarter.com/passive-house-tutorials/

But now no purchase can be completed and the potential buyer returns from PayPal to discover, instead of the video and other material, the message:

"There was a problem finalizing your purchase with PayPal. Please try again."
Our API details appear correct.
The main technical item that has changed recently is that the site has been moved to the sub-folder /2015-site
http://www.energyquarter.com/2015-site/

Could this be the issue?
We really need to resolve this as soon as possible, as customers cannot purchase our pay per view material.

Any help would be appreciated

Regards,
Estelle