If you were using Guzzle along from version 2.8.7 and recently upgraded to latest you may found some things different to set. Especially if you sign your client authentication with self certified authority for your SSL keys.
This is the change:
$client = new Client($config->getBaseApiUrl(), array( - 'curl.CURLOPT_SSL_VERIFYHOST' => false, - 'curl.CURLOPT_SSL_VERIFYPEER' => false + Client::SSL_CERT_AUTHORITY => false ));
Pairing up with @develCuy from drupal community, thanks for the tip!