When the Origin Content Server (OCS) returns a content-encoding response header of one type but encodes the data differently, the Web Security Service (ThreatPulse cloud) displays the exception page to forbid the web access.
Content Encoding Error
The web server whose HTTP response headers indicate that it will be responding with one type of encoding, responds instead with a different type of encoding.
For example, what is commonly seen is that a web server returns a gzip compressed object, but the content is actually text.
If we take a PCAP when accessing this site, you can see that most of the responses have "Content-Encoding: gzip".
We can see the response content is served in gzip (looks like garbage data).