NextCloud with nginx: subdomain config for NextCloud serves document root instead of folder

Question

I'm trying to set up nextcloud on nextcloud.mydomain.com while I have my regular website on mydomain.com. Here's my default.conf for nginx:

server {
  listen 80;
  listen [::]:80;
  server_name IP_HERE;
  root /var/www/html/;
  index index.php index.html index.htm index.nginx-debian.html;

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

  error_page 404 /404.html;
  error_page 500 502 503 504 /50x.html;

  location = /50x.html {
    root /var/www/html;
  }

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

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

The nextcloud.conf file follows Step 4 of this tutorial: https://www.linuxbabe.com/ubuntu/install-nextcloud-11-ubuntu-16-04-nginx-config

I don't know why, but when I access nextcloud.mydomain.com I just get the index.html in my /html webroot instead of the nextcloud setup wizard in the /nextcloud folder.

Any ideas? Thank you very much in advance!