Nextcloud can't connect to Collabora

i've setup a docker host, and wanted to setup nextcloud with collabora-office. But it simply does not find the collabora host. my compose file is the following:

version: '3.2'

services:
  db:
    image: mariadb:10.5
    restart: always
    volumes:
      - db:/var/lib/mysql
    environment:
      - MYSQL_DATABASE=xxx-nextcloud
      - MYSQL_USER=xxxnc
      - MYSQL_PASSWORD=evilcorp
      - MARIADB_RANDOM_ROOT_PASSWORD=yes

  nextcloud:
    image: nextcloud:20

    restart: always

    volumes:
      - nextcloud:/var/www/html
    environment:
      - MYSQL_DATABASE=vow-nextcloud
      - MYSQL_USER=xxxnc
      - MYSQL_PASSWORD=evilcorp
      - MYSQL_HOST=db:3306
    networks:
      - proxy
      - default
    labels:
      - "traefik.enable=true"
      - "traefik.http.routers.nextcloud.entrypoints=http"
      - "traefik.http.routers.nextcloud.rule=Host(`storage.xxx.org`)"
      - "traefik.http.middlewares.nextcloud-https-redirect.redirectscheme.scheme=https"
      - "traefik.http.routers.nextcloud.middlewares=nextcloud-https-redirect"
      - "traefik.http.routers.nextcloud-secure.entrypoints=https"
      - "traefik.http.routers.nextcloud-secure.rule=Host(`storage.xxx.org`)"
      - "traefik.http.routers.nextcloud-secure.tls=true"
      - "traefik.http.routers.nextcloud-secure.tls.certresolver=http"
      - "traefik.docker.network=proxy"
    depends_on:
      - db

  collabora:
    image: collabora/code:latest
    restart: always
    networks:
      - default
      - proxy
    ports:
      - 9980:9980
    cap_add:
     - MKNOD
    volumes:
      - /etc/timezone:/etc/timezone:ro
      - /etc/localtime:/etc/localtime:ro
    environment:
      - domain=storage\\.xxx\\.org
      - extra_params=--o:ssl.enable=false --o:ssl.termination=true
      - DONT_GEN_SSL_CERT=YES

    labels:
      - "traefik.enable=true"
      - "traefik.http.routers.office.entrypoints=http"
      - "traefik.http.routers.office.rule=Host(`office.xxx.org`)"
      - "traefik.http.middlewares.office-https-redirect.redirectscheme.scheme=https"
      - "traefik.http.routers.office.middlewares=office-https-redirect"
      - "traefik.http.routers.office-secure.entrypoints=https"
      - "traefik.http.routers.office-secure.rule=Host(`office.xxx.org`)"
      - "traefik.http.routers.office-secure.tls=true"
      - "traefik.http.routers.office-secure.tls.certresolver=http"
      - "traefik.http.services.nextcloud.loadbalancer.server.port=9980"

volumes:
  db:
  nextcloud:

networks:
  proxy:
    external: true

Can someone help me? When i activate collabora-app in nextcloud, and type in https://office.xxx.org as collabora address it says: "Can't connect to collabora Online Server"

The Server is up and running, in browser i can reach it with office.xxx.org and get an "OK" as response.

(as you might see, there is also a traefik-proxy container running on the docker host)

if your docker host is running ufw-firewall, try to: ufw allow https this helped me a lot ;)