PayPal Error: Sorry, an Error Occurred After You Clicked the Last Link

I’m just coming to the end of a project that required checkout via PayPal. The system is to be hosted in an iframe embedded in a third party site’s page, but when redirecting to PayPal the iframe ‘pops out’ to take over the whole page (PayPal forces this behaviour).
All was working smoothly using the ‘PayPal [...]