Hi there,
Our server doesn’t have any blocks in place when it comes to cURL (or no one would be able to activate their license key).
Sometimes (rarely) our server will block server IPs if it detects some sort of abuse, so we can check if that’s the case if you can share your server IP with us.
More often, it’s the cURL version or firewall in place on your server which blocks the communication. This post has some great info on this specific cURL error: https://www.samuelaguilera.com/post/curl-error-28-wordpress