I have the same issue; started Thursday 5th October after I changed my password and clicked to try the beta site. Now I get Flash Player attempting to load on any panel that uses it; chat, extensions, stats page, revenue.
I can no longer use chat or read chat on the official Twitch site, so have to select beta. Chat works in the desktop app and devices - this is purely a browser issue (Edge, Chrome, etc).
The issue occurred at exactly the same time on both my desktop and laptop - so nothing to do with anything I’ve installed (hadn’t installed anything anyway).
I would use dashboard in beta, but I can’t navigate to a beta version of dashboard etc.
I have submitted a bug report and even contacted Microsoft, in case it was my end.
I have read another post that indicates it might be the password. So will be changing mine to see if it is resolved after 24 hours passes.
Edit - Changing password, clearing cache (including passwords) worked straight away.