After investigations, commit the proposed patch by kklimonda presented with the following introduction : cm/templates/site/login_form.html template uses a relative url for the form action, which seems to break deployment in the subfolder when trailing slash is missing (the form redirects to the upper folder breaking login). Using {% url login %} (as in the attached patch) instead would fix the issue.

# django helpers collections
from django.conf import settings
from django.core.urlresolvers import reverse

def absolute_reverse(view_id, args):
    return settings.SITE_URL + reverse(view_id, args=args)