After you login with Public key, there was a problem to switch to root access with su – command.
When you type and change root password with...
Although it says root password has changed, it does not change (like a bug).
To solve this issue;
After you give a password to root with the command below
changing mode with '#chmod 4755 /bin/su' is enough to resolve the problem. Now, you can get access from your user account (logged in with the public key) to root access.
In my understanding, there is a permission issue on the directory placed above which does not allow us to change root password (Although it never says so).