[Pro Sites] Error on signup "You may have already paid!"...

Maybe I am doing something wrong... testing the Pro Sites plugin and cannot see to get the payment through on the signup page. I get the following error notification:

Whoops, looks like you may have tried to submit your payment twice. You may have already paid! Check your subscripton info below to confirm:

Repro steps:

- Login as superadmin
- Create a new subsite with a new subsite user as the admin
- Login as that new user to that new subsite (in different browser app, to keep my WP logins separate)
- Click on free trial nag/notification about checkout that is displayed at top of the dashboard pages
- On the checkout page that comes up I fill in credit card details (using PayPal test account) (note also the back-end uses the sandbox account as well)
- Get error mentioned above...

Ideas?

Also tried using a real credit card (and the sandbox as the processing back-end)... Notice the error message is in a div id="psts-general-error". Not sure if this error comes from PayPal or Pro Sites...