can't bind to socket: 0.0.0.0:443 Address already in use
Either you have another webserver installed that is listening on port 443 (serving HTTPS) already or you manually started a Lighttpd process.
Only another port in that conf-line:
$SERVER["socket"] == ":443"
You mean you changed that port in the Lighttpd config and your Pi-hole admin interface is reachable via HTTPS? Do you need to add that changed port to the URL? Otherwise a second webserver is running. Only strange would be then that the other webserver does not block port 80, too.
To get an overview about which process is listening on which port:
mod_openssl warning can be ignored, but you can mute it by adding
server.modules += ("mod_openssl") as first line to your