Can you try log in as localos/root. As per the release notes,
"In vSphere 5.5, vCenter Single Sign-On no longer supports multiple default domains. This might cause problems for users who attempt to log in to a vCenter Server system that is installed on Windows, or for users who log in to the vCenter Server Appliance.
Workaround: Include the domain prefix with the user name when you log in to a nondefault domain. For example, the user root on the local operating system can log in as localos\root. The user user1 in the domain DOMAIN1 can log in as DOMAIN1\user1.
This behavior is not considered a bug, but included in the release notes for completeness."
Regards,
Sree