Comment 5 for bug 1243187

Thanks Kymil-arcadia for pointing out my mistake - reading my own comment again I can't figure out the sense of my solution.

So here is the full story:
By default, /etc/openstack-dashboard/local_settings.py has the following lines:

LOGIN_URL='/horizon/auth/login/'
LOGIN_REDIRECT_URL='/horizon/'

It is necessary to change them to:

LOGIN_URL='/horizon/auth/login/'
LOGOUT_URL='/horizon/auth/logout/'
LOGIN_REDIRECT_URL='/horizon/'

Adding that one extra LOGOUT_URL that you also mentioned. Without that line, Horizon doesn't add the /horizon/ part of the URL, which is required.