Using Cloudflare for HTTPS, empty preview pane in edit page

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This is my nginx config:

server {
    listen 80;
    server_name     forum.exemple.com;

    location / {

        proxy_pass http://unix:/var/discourse/shared/standalone/nginx.http.sock:;
        proxy_set_header Host $http_host;
        proxy_http_version 1.1;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto $scheme;

        auth_basic "Staff only";
        auth_basic_user_file /etc/nginx/.htpasswd/.htpasswd-staff;

    }

    access_log  /var/log/nginx/forum_access.log;
    error_log  /var/log/nginx/forum_error.log;
}

The thing is, it’s CloudFlare who make the https redirect rules. So I have to listen the port 80 in nginx, else it doesn’t work…
The case “Force https” is not active in admin. Should I ?