wp-comments-post.php returns 504 on nginx

Setup a fresh droplet on Ubunut 16.04 and installed a basic LEMP stack with mariadb, php7, nginx. Installed wordpress to /var/www/mydomain.com/ using wp-cli. Created a basic new nginx configuration in /etc/nginx/sites-available/mydomain.com.

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

        root /var/www/mydomain.com;
        index index.php;

        server_name mydomain.com;

        location / {
                try_files $uri $uri/ =404;
        }

        location ~ \.php$ {
                include snippets/fastcgi-php.conf;
                fastcgi_pass unix:/run/php/php7.0-fpm.sock;
        }
}

When I use the website, it works perfectly except when I try to post a comment, it hangs for over a minute before nginx returns a 504 gateway time-out. However — comment is received.

This problem disappears when I set server_name _; and use this site as the default_server.

Tried php 7.1, wordpress 4.7, wordpress 4.8, ppa:nginx/stable package, etc, with no success.

Anyone else seen this problem? Cheers!