"Checkout with paypal" just refreshing the same page on pro sites payment page

When I sign up for an account and go to the mysite.com/pro-site/?bid=2 I choose the pro sites level that I want, and click the Checkout with paypal button, but it just refreshes the same page and presents me with the exact same options.

I tried disabling the theme (and not using the gravity forms sign up, just the native wordpress one) but that hasn't made any difference.

Regards,
Brendan