We have a vCenter 5.1 Appliance VM that seems to lose its Root password every few days and our backups start to fail. We try to log in as Root and get an "incorrect username or password" error.
We then have to log into an ESXi host and restart the vCenter VM. After the vCenter reboots, we can log into it again with Root.
If its vCSA 5.5, it is likely to be this,
For other versions there are few possibilities I can think of
Do you have any password management/compliance utility , these tools have a tendency to reset password regularly
Have you configured password aging
Interesting thing to note is that on a reboot the password is recovered, strange...