There was a problem finalizing your purchase with PayPal. Please try again.

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 promised material, the message:

"There was a problem finalizing your purchase with PayPal. Please try again."

Trying again does not produce a different result.

Is there a bug? What can we do? We've double checked the API details, everything appears to be in order, except that the final step does not succeed.