Usage:
$ pwconv

# Set new password for jacks which missing from /etc/shadow file
$ passwd jacks

passwd: Authentication token manipulation error

First check if your / filesystem is mounted read-write or read-only. Restart your machine and at the Grub prompt (tested on a fresh install of RHEL 6.2 with Grub version of 0.97), change ro to rw as indicated below :

kernel /vmlinuz-2.6.32-220.el6-x86_64 ro root=/dev/mapper….

kernel /vmlinuz-2.6.32-220.el6-x86_64 rw root=/dev/mapper….

An other solution is to make sure that the content of /etc/passwd and /etc/shadow are  the same with the pwconv command.

As root, before reinitializing again its passwd, run the command below :

root@localhost:~# pwconv

Then

root@localhost~# passwd

You should have no more error message.