Cannot find domain in the certificate - Let's Encrypt

I am trying to setup an SSL Certificate for mail.cd.mt, but everytime I try to generate the certificate I get Cannot find domain in the certificate. Furthermore, when I try to access the webmail via https://mail.cd.mt/ I get the Apache is functioning normally message.

What am I missing in my configuration?

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You are reading too much into the output … if you look, the certificate is issued and valid.

Webmail is accessed via https://webmail.cd.mt, not https://mail.cd.mt. The mail subdomain is used for postfix/dovecot - not for https.