I changed the faulty line in NginX default config file to:

server {
listen 80 default_server;
listen [::]:80;

root /usr/share/nginx/html;
index index.html index.htm index.php;

server_name mydomain.com www.mydomain.com;

location / {
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header Host $host;
proxy_pass http://127.0.0.1:8080;
}

location ~ /\.ht {
deny all;
}
}

So now, on hitting in the browser:
https://mail.mydomain.com

I get the error on the browser:
This webpage has a redirect loop
The webpage at https://mail.mydomain.com/ has resulted in too many redirects. Clearing your cookies for this site or allowing third-party cookies may fix the problem. If not, it is possibly a server configuration issue and not a problem with your computer.

The NginX error is gone but the Apache error remains the same.
I think it's some config problem with setup of iRedMail so I'm going to decommission Apache2 for iRedMail setup and move the entire iRedMail setup on NginX directly.