Having same problems with v11 CE and NGINX.  Our entire site is using https but Slides keeps bugging out with mixed content errors trying to show any presentation.

Mixed Content: The page at 'https://contextnetworks.net/slides/slide/context-networks-30-000-foot-3' was loaded over HTTPS, but requested an insecure resource 'http://contextnetworks.net/slides/embed/3?page=1'. This request has been blocked; the content must be served over HTTPS.

Our nginx config has the supposed proper entries (see below)

location / {
proxy_pass http://odoo;
# force timeouts if the backend dies
proxy_next_upstream error timeout invalid_header http_500 http_502 http_503;
# set headers
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forward-For $proxy_add_x_forwarded_for;
# Let the OpenERP web service know that we're using HTTPS, otherwise
# it will generate URL using http:// and not https://
proxy_set_header X-Forwarded-Proto https; 
# emphasis above is mine for this post
# by default, do not forward anything
proxy_redirect off;
}

How in heck to resolve?