It looks strange, but after changing the "root" password I cannot login anymore. I followed exactly the process according to Vsphere 5.5 reset root and I used copy-and-paste for the password but strangely it always says invalid password ?
SnapOverflow
OK the problem is a bug in VMWare ESXi 5.5 : If you change a password to something longer than 30 characters it will not complain but your password will be changed anyway to only the first 30 chars of the provided password ! To be able to login again, you have to use the first 30 chars of your password.
Possibly related. In esxi 6, changing the root password in the Vsphere client did not seem to properly take effect. I could no longer log into vsphere, but could still log into the machine console with the old password. Restarting all the management agents seemed to bring everything into line again.