Prosites http 302 on click of + Your Account

After installing and configuring Prosites, a site gets a "+ Your Account" button. In the network admin, the site is set to be manually extended permanent. I assume the button "+ Your Account" displays the "product grid" for upgrading (?) the prosite level. Whatever is on the page, it's blank:

"GET /softwareme/wp-admin/admin.php?page=psts-checkout HTTP/1.1" 302 20041

How can I find out what the page error is - I assume it's incorrect configuration, but not sure what setting. Is there any tutorial on setting up Pro Sites, especially with Membership? All the I have is the Plugin Usage page.

Thanks for any help,