After a recent upgrade to WordPress 4.6 had problems with cURL (eg when updating plug-ins that use cURL): cURL error 23: Failed writing body (373 = 449!)
Judging by the forum, to 4.6 cURL broke, and 4.6.1 are still not rolled out the fix.
The decision to fix the line:
if (!function_exists('curl_init') || !function_exists('curl_exec')) {
to
if (true || !function_exists('curl_init') || !function_exists('curl_exec')) {
in ./wp-includes/Requests/Transport/cURL.php file
I don't recommend to edit Wordpress core files but Wordpress Developer have to fix that in next update.