Websocket connection error

Hello together, We have setup planka in our company via docker on our synology. Configuration is as follows:

version: '3.9' services: planka: image: ghcr.io/plankanban/planka:latest container_name: Planka command: > bash -c "for i in seq 1 30; do ./start.sh && s=$$? && break || s=$$?; echo "Tried $$i times. Waiting 5 seconds..."; sleep 5; done; (exit $$s)" restart: on-failure:5 volumes: - /volume1/docker/planka/avatars:/app/public/user-avatars - /volume1/docker/planka/images:/app/public/project-background-images - /volume1/docker/planka/files:/app/private/attachments ports: - 3617:1337 environment: - BASE_URL=https://planka.ourwebsite.com - TRUST_PROXY=1 - DATABASE_URL=postgresql://postgres@postgres/planka - SECRET_KEY=xxxxx - DEFAULT_ADMIN_EMAIL=[email protected] - DEFAULT_ADMIN_PASSWORD=xxxx - DEFAULT_ADMIN_NAME=xxxx - DEFAULT_ADMIN_USERNAME=xxxx - NODE_ENV=production

depends_on:
  - postgres

postgres: image: postgres container_name: Planka-DB restart: on-failure:5 volumes: - /volume1/docker/planka/db:/var/lib/postgresql/data:rw environment: - POSTGRES_DB=planka - POSTGRES_HOST_AUTH_METHOD=trust

We have set a reverse proxy rule to 443 and to localhost 3617. We also set the websocket option in our synology.

Nevertheless one of our client just sees the loading screen from the startup an can't get any further. From the google debug consule they get an websocket error (sails.io)

Do you have any advice? We are loving planka, but we need to have to make it work for all of our clients. Thank you in advance, Matthias