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.
{% load i18n %}{% autoescape off %}
{% trans "You're receiving this e-mail because you requested a password reset" %}
{% blocktrans %}for your user account at {{ site_name }}{% endblocktrans %}.
{% trans "Please go to the following page and choose a new password:" %}
{% block reset_link %}
{{ protocol }}://{{ domain }}{% url django.contrib.auth.views.password_reset_confirm uidb36=uid, token=token %}
{% endblock %}
{% trans "Your username, in case you've forgotten:" %} {{ user.username }}
{% endautoescape %}