Apache SSL not working - server took too long to respond

Question

I am trying to get Apache (2.4.41, Ubuntu) to work with SSL and am not having luck. Whenever I visit the site in my browser, I get the error "This site can't be reached: my-domain took too long to respond."

I have a valid certificate and key, though I am not entirely sure which variant to use. I have tried several (both with chain and without) and can't seem to get it to work.

Here are the cert versions available to me:

  Available formats:
       as Certificate only, PEM encoded
       as Certificate (w/ issuer after), PEM encoded
       as Certificate (w/ chain), PEM encoded
       as PKCS#7
       as PKCS#7

    Issuing CA certificates only:
       as Root/Intermediate(s) only, PEM encoded
       as Intermediate(s)/Root only, PEM encoded

Apache itself works fine with http, no problem. I have set up a config which redirects http to https traffic, also works no problem. My SSL config is the default in Apache:

<IfModule mod_ssl.c>
        <VirtualHost _default_:443>
                ServerAdmin webmaster@localhost

            DocumentRoot /var/www/html

            ErrorLog ${APACHE_LOG_DIR}/error.log
            CustomLog ${APACHE_LOG_DIR}/access.log combined

          
            SSLEngine on

            SSLCertificateFile      /etc/apache2/certs/my-domain.crt
            SSLCertificateKeyFile /etc/apache2/certs/my-domain.key
            SSLCertificateChainFile /etc/apache2/certs/my-domain.ca

            <FilesMatch "\.(cgi|shtml|phtml|php)$">
                            SSLOptions +StdEnvVars
            </FilesMatch>
            <Directory /usr/lib/cgi-bin>
                            SSLOptions +StdEnvVars
            </Directory>
    </VirtualHost>
</IfModule>

I have checked that the hash of the .crt and .key match. (If I intentionally mismatch them, I get an error when starting Apache).

I do not get an error message in the Apache log. My ports.conf is listening on 443:

Listen 80
Listen 443

I have enabled the SSL config with sudo a2ensite default-ssl. a2enmod ssl says "Module ssl already enabled".

On the server, I checked whether port 443 is open using nc and it returns "Connection to my-domain 443 port [tcp/https] succeeded!"

What else could I test? I'm out of ideas.