I received an email recently regarding the above warning message. This occurs with an ESX host that has been installed without changing the root password.
It is a useful warning to ensure the root account is protected by setting a password.
To fix, login to the service console (SSH or local)
type su to obtain root privileges.
At this point, a new root password is prompted, enter the new root password as requested.
Now the password has been changed, to remove the warning message place the server into maintenance mode and disconnect the host.
From the service console or SSH session restart the management agents
service mgmt-vmware restart
service vmware-vpxa restart
Reconnect the host to the vCenter. The error message should no longer appear.