'Bad Gateway' on nextcloud, nginx proxy login

[Jedibooney]

When I attempt the first login to nginx proxy manager, following the nextcloud video instructions, I receive a bad gateway error. I'm using the following docker-compose.yml:

version: "3"

volumes:
  nextcloud-data:
  nextcloud-db:
  npm-data:
  npm-ssl:
  npm-db:

networks:
  frontend:
    # add this if the network is already existing!
    # external: true
  backend:

secrets:
  mysql_root_password:
    file: ./mysql_root_password.txt
  mysql_password:
    file: ./mysql_password.txt

services:
  nextcloud-app:
    image: nextcloud
    restart: always
    volumes:
      - nextcloud-data:/var/www/html
    environment:
      - MYSQL_PASSWORD_FILE=/run/secrets/mysql_password
      - MYSQL_DATABASE=nextcloud
      - MYSQL_USER=nextcloud
      - MYSQL_HOST=nextcloud-db
    networks:
      - frontend
      - backend
    secrets:
      - mysql_password

  nextcloud-db:
    image: mariadb
    restart: always
    command: --transaction-isolation=READ-COMMITTED --binlog-format=ROW
    volumes:
      - nextcloud-db:/var/lib/mysql
    environment:
      - MYSQL_ROOT_PASSWORD_FILE=/run/secrets/mysql_root_password
      - MYSQL_DATABASE=nextcloud
      - MYSQL_USER=nextcloud
    networks:
      - backend
    secrets:
      - mysql_root_password

  npm-app:
    image: jc21/nginx-proxy-manager:latest
    restart: always
    ports:
      - "80:80"
      - "81:81"
      - "443:443"
    environment:
      - DB_MYSQL_HOST=npm-db
      - DB_MYSQL_PORT=3306
      - DB_MYSQL_USER=npm
      - DB_MYSQL_PASSWORD_FILE=/run/secrets/mysql_password
      - DB_MYSQL_NAME=npm
    volumes:
      - npm-data:/data
      - npm-ssl:/etc/letsencrypt
    networks:
      - frontend
      - backend
    secrets:
      - mysql_password

  npm-db:
    image: jc21/mariadb-aria:latest
    restart: always
    environment:
      - MYSQL_ROOT_PASSWORD_FILE=/run/secrets/mysql_root_password
      - MYSQL_DATABASE=npm
      - MYSQL_USER=npm
    volumes:
      - npm-db:/var/lib/mysql
    networks:
      - backend
    secrets:
      - mysql_root_password