Coursepress Pro/Marketpress 'checkout' button goes to blank page and not payment gateway

I have some courses set up for purchase using Coursepress Pro/Marketpress.
Everything is working fine up until the checkout point.
You can add the course to your cart where you are taken to
There as soon as you hit the 'CHECKOUT' button, you get sent to a blank page at

You should be getting sent to Paypal right? All Paypal express info is set up and ready to go so not sure why it isn't sending the user to Paypal. Any thoughts?