6.5.1 use internal session files to resolve the cookie issue. Theses files should not store in root. Check your session directory in control panel - Primary Settings - Advanced Options
Set it to an existings directory (if you can set it outside the webroot) und do the test. it must have the permission 666 or 777. Leave your membersettings to cookies! not turn traking members with sessions. The session files should be deleted automaticly after ~30 minutes if no longer needed.
