The server committed a protocol violation. Section=ResponseStatusLine ERROR

Try putting this in your app/web.config:

<system.net>
    <settings>
        <httpWebRequest useUnsafeHeaderParsing="true" />
    </settings>
</system.net>

If this doesn't work you may also try setting the KeepAlive property to false.

Sometimes this error occurs when UserAgent request parameter is empty (in github.com api in my case).

Setting this parameter to custom not empty string solved my problem.

The culprit in my case was returning a No Content response but defining a response body at the same time. May this answer remind me and maybe others not to return a NoContent response with a body ever again.

This behavior is consistent with 10.2.5 204 No Content of the HTTP specification which says:

The 204 response MUST NOT include a message-body, and thus is always terminated by the first empty line after the header fields.