I now have a problem with completing a payment through PayPal Payments Pro. I receive a message to say "Invalid configuration" on submitting an order. I have a PayPal sandbox account and have created a test API account by creating a test PayPal Payments Pro account, and these details have been entered into the cart config settings, ensuring the shop is running in sandbox mode.
Any ideas what the problem may be, or a method for debugging to gain to some more verbose exception details?