The first time you access the portal after you deploy SSO, you must access the portal from the main portal URL: http://<servername>/portal/Server.pt.
If you try to access the portal through /portal/sso/SSOLogin.aspx (.NET) or /portal/SSOServlet (Java), your request fails and the following error appears in AquaLogic Interaction Logging Spy trace logs: The SSO Login Page was unable to retrieve the request URL from the session. Will use a relative redirect to return to the main page.