Could you please also print out the request variable, before this line $response = curl_exec($curlCh);?
Do you maybe use a proxy or is one configured in your config.inc.php?
A few files are shared between import/export plugins and they are solved as symbolic links at the moment, see for example ojs/DOIExportDom.inc.php at ojs-stable-2_4_8 · pkp/ojs · GitHub. Sometimes there are problems with those files when unpacking the release – they are not properly copied. I also think this shouldn’t be the problem here – because you probably couldn’t export the XML file either… but… I just don’t know why it is not working… :-\