I have developed some things with LiveView. All is working on my machine (how many times I would have said that …), and also on the test server. The problem is that it is not working on the production server. If I open Firefox’s developer tools, I can see an error message:
Firefox can’t establish a connection to the server at wss://mydomain.com/live/websocket?vsn=2.0.0
And after that, an HTTP failed request with code 426 Upgrade Required.
Is your production box behind a load balancer / reverse proxy of any kind? Maybe it doesn’t allow upgrade requests through, then cowboy would respond with 426 probably, since it expects one. But that’s just speculation.