Progress!
HTTPs showing an error is fine, you just need to replace the built in certs with your actual - purchased or free (via LetsEncrypt) - SSL certificates.
In the vhost conf replace:
SSLEngine on
SSLCertificateFile /etc/ssl/certs/ssl-cert-snakeoil.pem
SSLCertificateKeyFile /etc/ssl/private/ssl-cert-snakeoil.key
With (for traditional SSL providers):
SSLEngine on
SSLCertificateFile /path/to/ssl/cert.crt
SSLCACertificateFile /path/to/chain.ca-bundle
SSLCertificateKeyFile /path/to/ssl/key.key
Or (for LetsEncrypt)
SSLEngine on
SSLCertificateFile /etc/letsencrypt/live/DOMAIN_HERE/fullchain.pem
SSLCertificateKeyFile /etc/letsencrypt/live/DOMAIN_HERE/privkey.pem
Paste the output of your cron and how you edited it please.