If you are having a Proxy server, check the mandatory proxy settings are configured correctly.
Kindly make sure below things configured correctly.
/websocket - websocket support needs to be enabled in the proxy to upgrade a http connection to a websocket connection.
/main/ifsapplications/web/server/notifications - websocket support needs to be enabled in the proxy to upgrade a http connection to a webscocket connection.
Please refer the “Mandatory proxy settings” section in below documentation.