Owncloud behind netscaler and nginx proxy: PR_CONNECT_RESET_ERROR - docker

Question

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 https://cloud.mydomain.com returns a correct redirect to https://cloud.mydomain.com/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.