[CentOS] - manipulation token error

hi all,

I can't change root password because in my VPS the ssh console gives me:

$ ssh -p xxxx2 username at ipaddress
username at ipaddress's password: 
Last login: Thu Jan  9 11:46:58 2014 from dynamic-adsl-xxx.xxx.xx.it
WARNING: Your password has expired.
You must change your password now and login again!
Changing password for username.
(current) UNIX password: 
passwd: Authentication token manipulation error
passwd: password unchanged
Connection to xxxxxx closed.

how I fix this problem?
thanks in advance

regards