Are you seeing these errors after booting into single user mode? If not, then is your server booted normally? Are there messages the output of `dmesg` about i/o errors? [by TrevorH]
I have another system (Redhat 6) with this problem. We are not able to login at all the only access we have is single user mode.
When we try to change the root password, or password of a user we just added in single user mode we get the message unix_chkpwd unknown user ...