MarketPress PayPal Chained Payments error

Hello,
I am running MarketPress on a multisite system.

When I try and set up PayPal Chained Payments for a client, we are getting the following error, when we click the confirm payment button

There was a problem connecting to PayPal to setup your purchase. Please try again.
520003 - Authentication failed. API credentials are incorrect.

Strange thing is, PayPal Chained Payments doesn't require any API keys just the admins paypal mail address.

Any help with this would be great, using PayPal express and adding API keys is not an option.

Cheers,
D