Pay for View setup results in Security error with paypal

First time setting up Pay for View, enabled on site, setup global settings, paypal signature input and all saved.

Using selection to protect one Vimeo video – and it all appears as it should, except when trying to pay for the event, I get

There was a problem connecting to PayPal to setup your purchase. Please try again.

10002 – Security error – Security header is not valid

and no access or ability to pay for the protected material.

Any ideas on what to check or change to permit the transaction to take place?