Can't access vCenter 7.0.0c SSH after "tdnf upgrade" on the PhotonOS

I've upgraded the packages on a freshly installed appliance-managed vCenter 7.0.0c, PhotonOS, issuing the SSH command "tdnf upgrade" (it's like yum upgrade or apt-get upgrade).

The upgrade was successful, but after the reboot, I can no longer access the SSH shell, I typed the correct root/password, but instead of having access to the system, I'm prompted again for the password. After typing the password, for the second time, I got an "authentication failed" and the shell window closes itself.

Thanks

Maurizio