jwilder/nginx-proxy: no access to virtual host

Question

I have a NAS behind a router. On this NAS I want to run for testing Nextcloud and Seafile together. Everything should be set up with docker. The jwilder/nginx-proxy container does no work as expected and I cannot find helpful information. I feel I am missing something very basic.

What is working:

• I have a noip.com DynDNS that points to my routers ip: blabla.ddns.net
• The router forwards ports 22, 80 and 443 to my NAS at 192.168.1.11
• A plain nginx server running on the NAS can be accessed via blabla.ddns.net, its docker-compose.yml is this:

version: '2'
services:
  nginxnextcloud:
    container_name: nginxnextcloud
    image: nginx
    restart: always
    ports:
      - "80:80"
    networks:
      - web

networks:
  web:
    external: true

What is not working:

• The same nginxserver like above behind the nginx-proxy. I cannot access this server. Calling blabla.ddns.net gives a 503 error, calling nextcloud.blabla.ddns.net gives "page not found". Viewing the logs of the nginx-proxy via docker logs -f nginxproxy logs every test with blabla.ddns.net and shows its 503 answer, but when I try to access nextcloud.blabla.ddns.net not even a log entry occurs. This is the docker-compose.yml for one nginx behind a nginx-proxy:

version: '2'
services:
  nginxnextcloud:
    container_name: nginxnextcloud
    image: nginx
    restart: always
    expose:
      - 80
    networks:
      - web
    environment:
      - VIRTUAL_HOST=nextcloud.blabla.ddns.net

  nginx-proxy:
    image: jwilder/nginx-proxy
    container_name: nginxproxy
    ports:
      - "80:80"
    volumes:
      - /var/run/docker.sock:/tmp/docker.sock
    networks:
      - web

networks:
  web:
    external: true

The generated configuration file for nginx-proxy /etc/nginx/conf.d/default.conf contains entries for my test server:

# If we receive X-Forwarded-Proto, pass it through; otherwise, pass along the
# scheme used to connect to this server
map $http_x_forwarded_proto $proxy_x_forwarded_proto {
  default $http_x_forwarded_proto;
  ''      $scheme;
}
# If we receive X-Forwarded-Port, pass it through; otherwise, pass along the
# server port the client connected to
map $http_x_forwarded_port $proxy_x_forwarded_port {
  default $http_x_forwarded_port;
  ''      $server_port;
}
# If we receive Upgrade, set Connection to "upgrade"; otherwise, delete any
# Connection header that may have been passed to this server
map $http_upgrade $proxy_connection {
  default upgrade;
  '' close;
}
# Apply fix for very long server names
server_names_hash_bucket_size 128;
# Default dhparam
ssl_dhparam /etc/nginx/dhparam/dhparam.pem;
# Set appropriate X-Forwarded-Ssl header
map $scheme $proxy_x_forwarded_ssl {
  default off;
  https on;
}
gzip_types text/plain text/css application/javascript application/json application/x-javascript text/xml application/xml application/xml+rss text/javascript;
log_format vhost '$host $remote_addr - $remote_user [$time_local] '
                 '"$request" $status $body_bytes_sent '
                 '"$http_referer" "$http_user_agent"';
access_log off;
        ssl_protocols TLSv1.2 TLSv1.3;
        ssl_ciphers 'ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384';
        ssl_prefer_server_ciphers off;
resolver 127.0.0.11;
# HTTP 1.1 support
proxy_http_version 1.1;
proxy_buffering off;
proxy_set_header Host $http_host;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection $proxy_connection;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $proxy_x_forwarded_proto;
proxy_set_header X-Forwarded-Ssl $proxy_x_forwarded_ssl;
proxy_set_header X-Forwarded-Port $proxy_x_forwarded_port;
# Mitigate httpoxy attack (see README for details)
proxy_set_header Proxy "";
server {
    server_name _; # This is just an invalid value which will never trigger on a real hostname.
    listen 80;
    access_log /var/log/nginx/access.log vhost;
    return 503;
}
# nextcloud.blabla.ddns.net
upstream nextcloud.blabla.ddns.net {
                ## Can be connected with "web" network
            # nginxnextcloud
            server 172.22.0.2:80;
}
server {
    server_name nextcloud.blabla.ddns.net;
    listen 80 ;
    access_log /var/log/nginx/access.log vhost;
    location / {
        proxy_pass http://nextcloud.blabla.ddns.net;
    }
}

Why is this minimal example not working?