Problem upgrading CRM version

When I go to <subscription>, then <change edition> and the choose the Professional version (I am currently on the old Professional), I get the following error on a further empy screen:

Please <a href="http://crm.zoho.com/crm/ZohoCRMStoreServlet?isStringReq=true">Create an account</a> before making your purchase

Any idea what is wrong here? Of course I am logged in, so creating an account is not possible anymore...