After rebooting a RH6.1 system which has been working fine for several months, I suddenly can not log in as any user but root. There is no /etc/nologin file, I have updated initscripts, and most other packages which have errata associated with them. The error I get when logging in as non-root user on a console is: "input in flex scanner failed" If I log in as root and su (or su -) to the other account, all works well, so it seems to not be an authentication problem.
The message is not from login, but rather from /lib/security/pam_console.so, a PAM helper module used by /bin/login. You most likely have a damaged configuration. The simplest fix is to reinstall the pam package (pam-0.68-7 in Red Hat 6.1) using rpm -U --force pam-0.68-7.*.rpm and see whether that corrects the problem. If not, you'll have to examine your PAM configuration in /etc/pam.d/*. Look for random file corruption that can occur when a machine is rebooted after a crash or from a sudden power down.
No response from reporter - assumed resolved.