curl error 18 - transfer closed with outstanding read data remaining

The error string is quite simply exactly what libcurl sees: since it is receiving a chunked encoding stream it knows when there is data left in a chunk to receive. When the connection is closed, libcurl knows that the last received chunk was incomplete. Then you get this error code.

There's nothing you can do to avoid this error with the request unmodified, but you can try to work around it by issuing a HTTP 1.0 request instead (since chunked encoding won't happen then) but the fact is that this is most likely a flaw in the server or in your network/setup somehow.

I bet this is related to a wrong Content-Length header sent by the peer. My advice is to let curl set the length by itself.

Seeing this error during the use of Guzzle as well. The following header fixed it for me:

'headers' => [
    'accept-encoding' => 'gzip, deflate',
],

I issued the request with Postman which gave me a complete response and no error. Then I started adding the headers that Postman sends to the Guzzle request and this was the one that fixed it.

I had the same problem, but managed to fix it by suppressing the 'Expect: 100-continue' header that cURL usually sends (the following is PHP code, but should work similarly with other cURL APIs):

curl_setopt($curl, CURLOPT_HTTPHEADER, array('Expect:'));

By the way, I am sending calls to the HTTP server that is included in the JDK 6 REST stuff, which has all kinds of problems. In this case, it first sends a 100 response, and then with some requests doesn't send the subsequent 200 response correctly.