Jan 01, 2018 03:29 AM|Siva Krishna Macha|LINK
It may be a network proxy issue. Please see troubleshooting guidelines from Microsoft Docs here: https://docs.microsoft.com/en-us/aspnet/signalr/overview/testing-and-debugging/troubleshooting
Excerpt from the article:
"InvalidOperationException: Not a valid web socket request.
This error may occur if the WebSocket protocol is used, but the network proxy is modifying the request headers. The solution is to configure the proxy to allow WebSocket on port 80.
So, see if you are using a proxy and if so, does that allow WebSockets?
As a side note, you may also refer these discussions to see if there are any browser specific config issues: