Yes and yes.

I've tried that too. Ran into the same issue.

How can I debug this?

Thanks!

Thanks Paul.

I'd restarted before I tested.

I just checked my nginx log files (access and error), but I did not find anything related to my access.

The content of my /etc/nginx/sites-enabled/default.conf (00-default.cong not found) looks like this:

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

        # SSL configuration
        #
        # listen 443 ssl default_server;
        # listen [::]:443 ssl default_server;
        #
        # Note: You should disable gzip for SSL traffic.
        # See: https://bugs.debian.org/773332
        #
        # Read up on ssl_ciphers to ensure a secure configuration.
        # See: https://bugs.debian.org/765782
        #
        # Self signed certs generated by the ssl-cert package
        # Don't use them in a production server!
        #
        # include snippets/snakeoil.conf;

        root /var/www/html;

        # Add index.php to the list if you are using PHP
        index index.html index.htm index.nginx-debian.html;

        server_name _;

        location / {
                # First attempt to serve request as file, then
                # as directory, then fall back to displaying a 404.
                try_files $uri $uri/ =404;
        }

        # pass PHP scripts to FastCGI server
        #
        #location ~ \.php$ {
        #       include snippets/fastcgi-php.conf;
        #
        #       # With php-fpm (or other unix sockets):
        #       fastcgi_pass unix:/var/run/php/php7.0-fpm.sock;
        #       # With php-cgi (or other tcp sockets):
        #       fastcgi_pass 127.0.0.1:9000;
        #}

        # deny access to .htaccess files, if Apache's document root
        # concurs with nginx's one
        #
        #location ~ /\.ht {
        #       deny all;
        #}
}

My nginx is running:
service nginx status
● nginx.service - A high performance web server and a reverse proxy server
   Loaded: loaded (/lib/systemd/system/nginx.service; enabled; vendor preset: enabled)
   Active: active (running) since Mon 2020-07-27 10:42:28 UTC; 2 days ago

Here is the content under /var/www/html:
root@mail:/var/www/html# ls
index.nginx-debian.html

I had a similar issue on one of my installs because ipv6 was disabled. The nginx install didn't like that and the iredmail nginx portion scripts in iredmail didn't run because of that. I manually installed nginx before installing iredmail and then disabled the ipv6 stuff in the nginx conf files. After doing that, iredmail installed just fine.