This is a common issue caused by the OpenSSL update a few months ago. The only way you can fix this is by disabling SSL Verifying. If you were using file_get_contents I could give a solution that works. However I rarely use cURL, so I can only provide what the internet tells me:
Try looking at this:
I found this on a Github Support page:
You can force Jetpack to communicate via HTTP only, by adding this constant to your site’s wp-config.php file:
define( 'JETPACK_CLIENT__HTTPS', 'NEVER' );