Matthias,

Can we get some help understanding the root cause here?

Unfortunately, the traceback does not tell anything useful.

First rule, to get a more usable trace would be to enable debug:
/etc/openstack-dashboard/local_settings 
set DEBUG = True
and restart httpd

Probably unrelated, there has been a rc2 release, which had many bugs fixed. I strongly recommend to test that version.

Created attachment 1083298 [details]
debug trace.html

Looks like we are configuring memcached for horizon incorrectly. I verified that if I put a single value for CACHES['LOCATION'] like:

CACHES = {
    'default': {
        'BACKEND': 'django.core.cache.backends.locmem.LocMemCache',



        'LOCATION': '192.0.2.18:11211',


    }
}


I was able to get a login page.

Extracting from debug trace what does not work:
CACHES	
{'default': {'BACKEND': 'django.core.cache.backends.locmem.LocMemCache',
             'LOCATION': ['192.0.2.18:11211',
                          '192.0.2.17:11211',
                          '192.0.2.19:11211']}}

Notes from IRC:
python-django-1.8.4-1.el7.noarch
https://docs.djangoproject.com/en/1.8/topics/cache/
<trown> gfidente: confirmed setting .memcached.MemcachedCache allowed it to start with list
<gfidente> should be https://review.openstack.org/235418

CACHES = {
    'default': {
        'BACKEND': 'django.core.cache.backends.memcached.MemcachedCache',
        'LOCATION': '127.0.0.1:11211',
    }
}

is correct, but be sure to install
python-memcached

Move to modified only when build is done in CBS.