Marketpress no longer working with Paypal Express Checkout

I have the latest update of Marketpress installed on 2 Multisite installations. I've been using Paypal Express Checkout and everything was working fine in live mode but now the handover from the shipping information page to Paypal is failing.

I've tried using sandbox credentials in sandbox mode and live credentials in live mode with the same results.

I also have Appointments+ installed on one of the multisite installations and that is working fine with sandbox and live credentials so I think it must be a Marketpress problem.

Any help with this would be much appreciated.