Matomo behind reverse proxy https --> http

We are using Matomo (version 3.9.1) behind a reverse proxy https --> http.
The reverse proxy is not managed by us but it seems to be configured correctly.
We have an issue with the login page. We use the reverse proxy mapping address and after entering the login & password we have the error “can’t reach this page” because it tries to go in https to our backend server. Simply making “back” redirects us to the https address of the reverse proxy and we are connected, wecan work normally … Matomo works, visits are tracked, we have access to the administration … but we would like to get rid of this issue …
We have configured matomo according to the article: How do I configure Piwik when the server is installed behind a proxy? FAQ - Analytics Platform - Matomo
The reverse proxy administrators ensure us they have correctly setup the mapping and redirections from https to http, and they assume somewhere in matomo the server address with https is hardcoded for the login actions.
Could you please advice on this issue ?
Thanks in advance.