I'm hoping I've resolved this issue. There was a bug in Edge (and possibly some version of IE) that led the websocket connections to be blocked by the browser after I added some extra security settings to the server. Specifically the bug lodged against Edge here:
https://developer.microsoft.com/en-us/microsoft-edge/platform/issues/10894548/I've added a workaround that looks like it allows the version of Edge I tested on to work, so please let me know if it works ok for you now too.
Regards,
Richard.