Pro Sites: Can not upgrade, page loops when paypal button is pressed


I have a membership site:

the general signup process is handled by "membership" and allows for registration on the site and to create a blog for free.

The Pro sites then takes over and allows for two premium upgrades. I have checked all settings and as far as I can tell... everything is correct.

The prosite upgrade page is accessible (i have already trashed it and re-cretaed it) and paypal is configured in the admin.

You get to the page and can select a plan (it highlights in red) but when you click the paypal button it loops to the page and displays a error at the top of the page:

"Please choose your desired level and payment plan."

Over and over again.

Please advise what I am doing wrong. feel free to go to the process your self, since its free and let me know if you can get past that page.

I have disabled all but the basic plugins.

Thanks for the help.