I searched other topic have same issue to me. I already enable Websocket protocol but it is not working. My current EPiServer is 11.13. Do you have any idea to resolve this features?
Have you tried reinstalling the complete IIS module?
and make sure you enabled below items
It seems that you might be seeing the bug that was fixed in CMS.UI 11.20.1 https://nuget.episerver.com/package/?id=EPiServer.CMS.UI&v=11.20.1
Hmm I see to have a similar problem, but enabling Websocket protocol on the server(s) did not work and CMS.UI is of version 11.25.0 so seems recent enough.
Just making sure when you say enabling WebSocket on the servers doesn't work - did you try the WebSockect functionality locally on the servers or did you try it using some public address?
If using some public address www.your-site.org then the traffic is going thrue your load balancer and what ever there is in your network (infra) - are you sure your infrastucture supports WebSocket protocol all the way? So that there is nothing in the infrastructure that blocks it.
I would first check on the servers locally that WebSocket protocol works (unless already done) and then check the infrastucture. For example in the past we've had issues with F5 load balancer where WebSocket was not enabled for some reason (some documentation says it should be enabled by default but it could be it has been disabled because nothing has used it).