Pro Sites upgrade not working

I have installed Pro Sites and entered my PayPal settings, and created a new site on a subdomain.
So far everything worked fine. But when I click on the yellow "You have 15 days left in your Pro free trial. Checkout now to prevent losing Pro features" link at the top of the new site's dashboard, it takes me to "/pro-site/?bid=2" which is a login page with the message "You must first login before you can choose a site to upgrade". But I'm already logged in...???

If I enter the new sites login details and try to login, the page just refreshes.

Can someone please tell me what's wrong.

Thanks for helping!
/Jens