Hello,
The problem is still present, we have switched the site to HTTPS hoping that the problem will be solved but it is still present. The problem does not seem to have any link with the https.
The url of the site is https://elearning.bb-solutions.wordpress07.pymac.fr/, indicate pymac as login and password for the htaccess.
The hosting uses a reverse proxy, but the site accepts it without problem.
Have a nice day,
Yann