vSphere 5.1 and 5.5 require SSO to be installed. I have found when troubleshooting login issues, you can login using the default SSO credentials you setup.

In vSphere 5.1 the SSO Administrator username is: admin@system-domain

In vSphere 5.5 the SSO Administrator username is: Administrator@vsphere.local

So for example, if you are using ESXi 5.5 and need help logging into vCenter, try logging in with the username: Administrator@vsphere.local and use the SSO password you used during the install/upgrade of vCenter. This user has Admin rights to your vCenter environment.

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