System Only Allows Root Logins

You probably have some permission problems, or you have a file
/etc/nologin.

In the latter case, put "rm -f /etc/nologin" in your /etc/rc.local or
/etc/rc.d/* scripts.

Otherwise, check the permissions on your shell, and any file names
that appear in error messages, and also the directories that contain
these files, up to and including the root directory.