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.

{% extends "site/layout/base.html" %}
{% load i18n %}
{% block content %}
<h1>500: {% blocktrans %}Server Error{% endblocktrans %}</h1>

{% blocktrans %}An error has occurred.{% endblocktrans %}
{% endblock %}