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.

{% autoescape off %}
{% block content %}
{% endblock %}
--
{{ CONF.workspace_name }}
{{ CONF.site_url }}
{% endautoescape %}