vSphere 5.1 has admin@system-domain as the default admin account. vSphere 5.5 changes that to administrator@vsphere.local. However, if you upgrade from 5.1 to 5.5 the default admin account remains admin@system-domain. Which is fine and dandy until the password for this account expires. Then you are unable to reset or login! See below. :)
Trying to login as usual
Password has expired, needs a reset
Reset fails though coz you can only reset for the vsphere.local domain
Missed out on taking a screenshot but if you were to try and login with administrator@vsphere.local instead you get an error that the credentials are invalid (because that account doesn’t exist!). So you are stuck!
What do you do?
Solution is to reset the admin password
When you do this vSphere automatically creates the administrator@vsphere.local account. Follow the steps in this KB article.
Now you can login with administrator@vsphere.local and the generated password.