socket io failed to connect but not sure what's the problem

[DeoLeung]

Describe the bug
GET /socket.io/?EIO=4&transport=websocket HTTP/1.1" 400 -

get 400 on server, and error in the front, with no debuggable information

Version
0.50.19

How did you install?

Docker
i have already config nginx to support websocket

To Reproduce

deploy arch: nginx -> nginx -> traefik -> changedetection(from sample docker, on linux)

Expected behavior
could see what's the problem in browser

Screenshots

[dgtlmoon]

I'm not sure, we dont officially support traefik (I have no idea what it is), but I would say its something todo with the websocket protocol and traefik

you should add more data from the chrome logs if possible

[DevrMichael]

The 400 response usually means the WebSocket upgrade request isn’t reaching the app correctly. In multi-reverse-proxy setups (nginx → nginx → traefik → container), one proxy often drops or rewrites the upgrade headers.

Things to check:

Make sure every proxy in the chain passes through:

proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";

In Traefik, confirm websocket is enabled on the route/service and that it’s forwarding to the correct internal port (/socket.io/ path).

If you’re behind HTTPS, verify that the backend container expects wss:// and not plain ws://.

You can also open the browser devtools → Network → WS tab to see if the upgrade handshake returns any details before 400.