Paypal Checkout Button just refreshes page (Supporter / Pro Sites)

http://yachtblogs.com/pro/?bid=21

The Paypal Checkout button just refreshes the page. Nothing else happens.

I've turned on IPN. I've confirmed that I entered the URL correctly, and my IPN history is empty. I've refreshed my API key info and copied it in several times.

I'm not sure what the catch is here.

Any ideas? Thank you!