Website on SSL via proxy but backend mediafileupload error XMLHttpRequest

I would not recommend using the admin interface through a proxy. It's much better to use a host file (just Google that) so you can hit the actual server itself. What we do is make an alias for the site like preview. Or staging. Or direct. And art that as a site domain alias and edit the site through that.

I assume the proxy is a cdn?

I learned something, never saw that article, i'll be doing that from now on as well then!

Fixed the issue by implementing the code on this page and then changing the line below: https://docs.kentico.com/k10/securing-websites/deploying-websites-to-a-secure-environment/configuring-ssl/ssl-accelerator-support

Change string forwardedSSL = headers.Get("HTTP_X-Forwarded-Proto");

To string forwardedSSL = System.Web.HttpContext.Current.Request.ServerVariables["HTTP_X-Forwarded-Proto"];