Hello
You should run tshw wizard on both Portal and xMII server. Refer sap note 1332726 Troubleshooting Wizard. You need to use example 3 for selecting log locations. While the tracing wizard is running on both portal and xMII server, you need to perform SSO authentication. After the authentication fails, analyze the traces generated on both system. If you are able to draw any meaningful information, upload the traces here and I will check them for you.
Regards,
Tapan