AutoSSL shows "No key ID has been set" using Let's Encrypt

Symptoms

AutoSSL shows the following error while trying to request a new SSL certificate.

No key ID has been set. Either pass “key_id” to new(), or create_account().

The cPanel error log also contains messages similar to the following.

warn [xml-api] Net::ACME2::LetsEncrypt->get_terms_of_service(): Net::ACME2::X::HTTP::Network: The system failed to send an HTTP “GET” request to “https://acme-v02.api.letsencrypt.org/directory” because of an error: Cpanel::Exception::Timeout/(XID 3bkkcq) 
at /var/cpanel/perl/Cpanel/SSL/Auto/Provider/LetsEncrypt/ToSCache.pm line 89.
Cpanel::SSL::Auto::Provider::LetsEncrypt::ToSCache::__ANON__(__CPANEL_HIDDEN__) called at /usr/local/cpanel/3rdparty/perl/532/lib/perl5/cpanel_lib/IO/Socket/IP.pm line 751

Description 

This issue is caused by a firewall blocking the connection between your server and Let's Encrypt's API server, acme-v02.api.letsencrypt.org.

Workaround

1. Access the server's command line as the 'root' user via SSH or "Terminal" in WHM.
2. Run the following commands to determine if the firewall is blocking the IPv4 or IPv6 connection.

   [root@server ~]# curl -I https://acme-v02.api.letsencrypt.org/
   curl: (7) Failed to connect to acme-v02.api.letsencrypt.org port 443: Connection timed out

   [root@server ~]# curl -I6 https://acme-v02.api.letsencrypt.org/
   curl: (7) Failed to connect to acme-v02.api.letsencrypt.org port 443: Connection timed out

   Please note a successful connection will show an HTTP response code.
3. Open outbound TCP port 443 for IPv4 or IPv6, depending on which commands failed to produce an HTTP response code.

Additional resources

What ports should I open in my network firewall?