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.

def update(obj, kwargs):
    """
    Update obj attributes with values from kwargs
    """
    for k,v in kwargs.items():
        if hasattr(obj,k):
            setattr(obj,k,v)