WebSocket Connection Error for some users?

Hello everybody,

my company uses shinyproxy for production and it worked well so far, but now we encountered a problem we have not seen before.

They receive the following 403 Error.

403

The use the latest browser and the app also works for us, the logfiles show no error. Is this problem caused by shinyproxy, nginx or some browser related issue? I assume it has to do something with their firewall, which blocks the websocket connection, but I know almost nothing about these security related issues. Can anyone help me?