In my case it happened when adding a new application to IIS that needed to be proxied with Apache.
I needed to set ProxyHTMLEnable On
to ProxyHTMLEnable Off
to get the content. (It looks like some js code most have triggered the error, but this gave me at least something to work from.)
IIS is sending the page content Content-Encoding: gzip
<Proxy "http://192.168.1.1:81">
ProxyHTMLEnable On #> change this to Off
ProxyHTMLURLMap ... ...
</Proxy>