[CentOS] cannot rewrite shadow password file

[Samuel Rochas]

Hi,

Before I had this problem, I've changed the root password twice, and 
something went wrong, since I was not able to login as root anymore :(

I've started the box with the GRUB option init=/bin/bash, mounted the 
disk rw and executed the passwd command successfully.
I've restarted and could log into the box as root. But then, trying to 
issue the passwd command again did not work.

I am not sure if and how those actions can be related to my problem?!?

Regards
Samuel