We have seen this problem a lot in 9.1.X. Usually resetting the security settings will fix it. There are also some security settings that may need to be turned back on. It is covered in tech tip 282.
https://downloads.avaya.com/css/P8/documents/101013036
Also be aware it takes a minute or two to properly connect, so have a little patience.
If you watch it in monitor, with HTTP setting on, when it is working properly you will see
11:24:52 9002797mS HTTP: WebSocket 172.25.112.20(51379)|VMailPro_Support: Sending Ping
11:24:52 9002799mS HTTP: WebSocket 172.25.112.20(51379)|VMailPro_Support: Received Pong
11:24
The hand shake that makes it work.