How to force D8 "Configuration synchronization" to send HTTPS download URL I have a Drupal 8 site hosted on an Apache Server.
There are 2 virtuals hosts : 


one listening on port 80, which redirects everything to HTTPS
one listening on port 443 which serves the htdocs directory where the Drupal site is located


Everything works well, except for the "Configuration synchronization" full export. When I click the "Export" button, I'm redirected to the home page, without any message or log.

Enabling the console on my web browser, I saw these calls :

POST https://my.domain.fr/admin/config/development/configuration/full/export
[HTTP/1.1 303 See Other 86 ms]
GET http://my.domain.fr/admin/config/development/configuration/full/export-download
[HTTP/1.1 301 Moved Permanently 101 ms]
GET https://my.domain.fr/
[HTTP/1.1 200 OK 183 ms]


The download process of the config file seems to be killed by the redirection.

The second URL (GET http) is sent by Drupal in the response to the first call, in the "Location" header.

How can I force Drupal to send only HTTPS url, even in the Location header in this precise case ?

Edit: Potentially linked post

Edit2: I'm having the same trouble with the RSS link on the site : on the page returned by https://my.domain.fr, the RSS link is http://my.domain.fr/rss.xml

Edit3: Found the solution for the export problem (see answer below). Still not a solution for the RSS link, but it's not the main subject of this post.
I forgot the SSLEngine On Apache directive in the configuration of the VHost listening on port 443.

With it, the Location header sent by Drupal contains now https scheme.