Horizon is built on top of Django. Django 1.5 makes the ALLOWED_HOSTS setting mandatory as a security measure.
Also, recent changes in PackStack cause this variable to be set when using Django 1.4.
The consequence is that the Horizon Dashboard only becomes accessible when using one of the host names explicitly defined in ALLOWED_HOSTS. Otherwise, an "error 500" occurs.
Workaround: Update ALLOWED_HOSTS in /etc/openstack-dashboard/local_settings to include the hostname(s) you wish to use for the Dashboard. Then restart httpd.
The dashboard will now work when accessing it using the desired hostname(s).