Reverse cache not working on start pages

[Dingo]

Hi!

I am unable to get reverse cache working on startpages. I am using Ubuntu
16.04 with everything updated. I have tried this example: 
https://www.nginx.com/resources/wiki/start/topics/examples/reverseproxycachingexample/

http {
    proxy_cache_path  /data/nginx/cache  levels=1:2    keys_zone=STATIC:10m
    inactive=24h  max_size=1g;
    server {
        location / {
            proxy_pass             http://1.2.3.4;
            proxy_set_header       Host $host;
#            proxy_cache            STATIC;
            proxy_cache_valid      200  1d;
            proxy_cache_use_stale  error timeout invalid_header updating
                                   http_500 http_502 http_503 http_504;
        }
    }
}

It works fine with the row "proxy_cache STATIC" disabled, but when I enable
it, I can not get any startpages woking. If I use
http://asiteonmyiisserver.com, it will only show the default IIS page, but
if I type: http://asiteonmyiisserver.com/idex.html it will work. What also
works is http://asiteonmyiisserver.com/products, and so on.  I also have one
other server that only hosts one site (apache server) with no host headers
involved, and that one works even with the startpage.

I am totally lost when it comes Nginx, so this could be something really
simple. I have searched a lot on the Internet and only found one guy with
the same problem. The server pointed to by proxy_pass is an IIS-server
hosting mostly wordpress domains on a single IP using host headers.

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,276670,276670#msg-276670