ProSite subscription reloads the page when the PayPal button is clicked

It seems like everything is set up well for Pro Sites integration with PayPal. However, when I click on the PayPal link after choosing a subscription, it simply refreshes the subscription page.
API Username, API Password and Signature are all there. There are no references to any transactions or errors in PayPal from my attempts. I'm using PayPal Express.

Any suggestions?