You have enabled streaming for this session in Fiddler (e.g. the STREAM button is depressed on Fiddler's toolbar, or it's a audio/video response and you have streaming enabled for those).
You need to move your code to either OnBeforeRequest
and remove the line where you try to set the response body, or you need to configure bBufferResponse=true on this session in one of those two earlier handlers.
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