If it helps anyone this fix worked for us Make sure you have https on your return fields eg input name= return type=hidden value=https://yourdomain.com/thanks-for-your-payment and input name= cancel_return type= hidden value=https://yourdomain.com/cancel-payment We had this setup years ago with plain http links. It silently failed last week with this stupid and uninformative vague error. Cmon Paypal! NB characters stripped for clarity eg quotes and html
... View more