the admin configured the proxy similarly as shown in the manual

In your shoes I would be double-checking this. Most of what you have told us here is symptomatic of either the ProxyPassReverse not being set or the proxy_http and/or proxy_html modules not being loaded or correctly configured. I note that the doc to which you linked makes no mention at all of the proxy_html module. You can check by simply replacing your entire application with a light webserver which just sends back a static page referencing a stylesheet reachable by an absolute path. If that fails to load the stylesheet then the problem lies with the proxy and not with your application/framework.