password problem --- interesting

["Jephe Wu" <jwu globalsources com>]

Hi,
I am using Redhat Linux 6.1  , after running a while , I found I can not change my user id password although 'passwd' had reported that password is updated successfully. 
in /etc/passwd , my password field always display a "*" character, so I can not login . 
I had run 'pwunconv' to disable my /etc/shadow .
why?
at the same time, I also can not add a new users, saying 
-------
[root sh-proxy /etc]# /usr/sbin/useradd test
useradd: unknown gid 100
[root sh-proxy /etc]# tail -1 /etc/passwd
test:!!:506:506::/home/test:/bin/bash
------

thanks
Jephe