Hi Frédérick and welcome to the Mattermost forums!
I’m not sure but I think this will only work if Mattermost is also running with https, since you said that you’re using Mattermost behind an nginx proxy with a self signed certificate I’m wondering why your SiteUrl and mattermost_exteernal_url still point to the http version - did you already try to switch to https and see if you can reproduce the problem?