Docker-owncloud behind nginx behind netscaler gives error PR_CONNECT_RESET_ERROR in firefox

I try to install a dockerized owncloud instance behind a nginx server (on the same virtual machine). The whole vm-server (redhat8) is behind a netscaler (type unknown, managed by my company) which handles the ssl handshake via wildcard certificate. The nginx server is reached only via port 80 http. My nginx-conf:

server {

    listen 80;
    server_name cloud.mydomain.com;

    location / {

        client_max_body_size    16000m;

        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_redirect  off;
        proxy_set_header  X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_pass http://localhost:8082/;

    }

}

The docker-compose.yml in use:

version: "3"

services:
  owncloud:
    image: owncloud/server
    container_name: owncloud_server
    restart: unless-stopped
    depends_on:
      - mariadb
      - redis
    ports:
      - "8082:8080"
    environment:
      - OWNCLOUD_DOMAIN=cloud.mydomain.com
      - OWNCLOUD_DB_TYPE=mysql
      - OWNCLOUD_DB_NAME=owncloud
      - OWNCLOUD_DB_USERNAME=owncloud
      - OWNCLOUD_DB_PASSWORD=owncloud
      - OWNCLOUD_DB_HOST=mariadb
      - OWNCLOUD_ADMIN_USERNAME=admin
      - OWNCLOUD_ADMIN_PASSWORD=********
      - OWNCLOUD_MYSQL_UTF8MB4=true
      - OWNCLOUD_REDIS_ENABLED=true
      - OWNCLOUD_REDIS_HOST=redis
      - OWNCLOUD_OVERWRITE_CLI_URL=https://cloud.mydomain.com
      - OWNCLOUD_OVERWRITE_PROTOCOL=https
      - OWNCLOUD_OVERWRITE_HOST=cloud.mydomain.com
      - OWNCLOUD_TRUSTED_PROXIES=0.0.0.0/16
      - OWNCLOUD_DEFAULT_LANGUAGE=de
   healthcheck:
      test: ["CMD", "/usr/bin/healthcheck"]
      interval: 30s
      timeout: 10s
      retries: 5
    volumes:
      - ./files:/mnt/data
    networks:
      - default
  mariadb:
    image: mariadb:10.6
    container_name: owncloud_mariadb
    restart: unless-stopped
    environment:
      - MYSQL_ROOT_PASSWORD=owncloud
      - MYSQL_USER=owncloud
      - MYSQL_PASSWORD=owncloud
      - MYSQL_DATABASE=owncloud
    command: ["--max-allowed-packet=128M", "--innodb-log-file-size=64M"]
    healthcheck:
      test: ["CMD", "mysqladmin", "ping", "-u", "root", "--password=owncloud"]
      interval: 10s
      timeout: 5s
      retries: 5
    volumes:
      - ./mysql:/var/lib/mysql
    networks:
      - default


  redis:
    image: redis:6
    container_name: owncloud_redis
    restart: unless-stopped
    command: ["--databases", "1"]
    healthcheck:
      test: ["CMD", "redis-cli", "ping"]
      interval: 10s
      timeout: 5s
      retries: 5
    volumes:
      - ./redis:/data
    networks:
      - default

This happens: a request to this link returns a correct redirect to Cloud mydomain login. But this page shows on Chrome a ERR_CONNECTION_RESET, under firefox a PR_CONNECT_RESET_ERROR. The owncloud server log shows multiple GET /login HTTP/1.0" 200 entries.

I play with the configuration now for days and ran out of ideas. Any help appreciated. Thanks.