Since I've installed Ubuntu-eee on my EeePc 900, i've notice a strange behaviour of the X server. Each time I locked my session, i was unable to unlock it with my right password while I could connect on a new session…  

Digging the google ground (the net…) i found this post that save myself and my eeepc (an my mind sanity 🙂

All the problem rely on a simple file permission issue !

Just correct the file permission of the unix_chkpwd command with the following command lines:

chown root:shadow /sbin/unix_chkpwd
chmod 2755 /sbin/unix_chkpwd

That's all.

Thanks to michaelkahl whose solve the problem.