i have the same Problem. I find out that the Problem is the Authentification via PAM (Pluggable Authentication Modules) but i don´t know how it can be fixed.
The Configuration File for pam is /etc/pam.d/nails.
I found that, starting with a RHEL 6 x86_64 minimal install, the following additional packages (plus dependencies) are required:
unzip, ed, pam.i686, libstdc++.i686
Though the installation instructions mention the pam.i686 requirement, they do not mention the other requirements.
if libstdc++.i686 is not installed, the installation will complete but you will receive "authentication failure" when you login to the web interface.