The composed request you are executing is missing the User-Agent header, which all browsers add by default. Fiddler Everywhere also adds the header with a default value, so I guess the header was explicitly removed during the test process. The server is unhappy with the missing header, which leads to the internal server error and the received status code. The solution is to introduce the User-Agent header, and you will be able to execute the request as expected.

While the User-Agent header is optional (as stated in RFC2616), some servers might be configured to not work without it to avoid (or at least try) spoofing and incompatible content being passed to a client that don't understand it. Virtually all modern-day browsers (and most desktop applications making HTTP requests) add the User-Agent header. So it is safe to say that the User-Agent is more or less widely used and not so much an optional header.

That said, it looks like the server behind that specific domain is not handling responses that lack the User-Agent header, which produces the 500 status error.