I’m now seeing something similar. A functional install where twitter login just stopped working for me as an admin. I’m not sure if this is affecting all users or just me though.
The error I get when trying to login is “An error occurred when starting authorization. Please try again.”, which is not very descriptive.
It seems like a POST request to https://discourse.wicg.io/auth/twitter is getting redirected to /auth/failure?message=request_error. Clearing Twitter’s cookies didn’t seem to help..