Customer can't check out through PayPal on Pro Sites

Hi,

My customer who want's to sign up after a trial period is having problems checking out with PayPal.

This is her site: http://responsetherapies.yourreikipractice.com/

When she clicks on the checkout button the page simply refreshes.

I have exactly the same issue when I create a site and then try and upgrade through PayPal as you can see here:
http://screencast.com/t/VDURZMfY

Please tell me how we can fix this issue as soon as possible.

It may be related to a similar issue being looked at: https://premium.wpmudev.org/forums/topic/customers-site-deactivated-payment-issue

Hope to hear soon.

Justin