Pro Site Upgrade is not working

I did setup the Pro Sites Plugin with module "Pay to Blog":

A new site owner can login at the dashboard of his trial site and everything is working, except that he can not upgrade to the Pro Level.

There is a link at the top saying: “You have 15 days left in your Pro free trial. Checkout now to prevent losing Pro features”
Also there is a “Your Account” button at the upper right and and at menu of the dashboard. But all 3 links generate only a 404 error.
I remember they where leading to a page with the upgrade buttons and a button for PayPal Express payment.

Why is that page no longer showing up?
Thanks for your help