I'm having a similar problem with a specific website. The web request is proprietary (API) and includes client credentials so I can't really post it here.
When I send the request to the url via a .NET application, it communicates fine, receives the result and displays it to me without any issue. However when I attempt to view the response data through Fiddler, I am prompted with the following message:
"Fiddler: UnGZip failed
The content could not be decompressed.
The magic number in GZip header is not correct. Make sure you are passing in a GZip stream"
This is on latest version of Fiddler (126.96.36.199002).
I have talked to the developer of the website, and they have suggested they are setting some headers improperly, but aren't sure how to fix on their end. I'm not sure what the issue is exactly. Is there a way to "relax" the error checking on the GZip in the same way that allows .NET and web browsers to load the content fine without any errors?