I followed the steps @sam listed, and got the popup error:
But I did manage to reproduce the OP. The error very specifically happens when the auth tokens expire on the server. If you delete the cookies on the client, it works fine. Recategorizing this to Contribute > Bug
To repro easily, set session length 1 hour, and log in via SSO. Fake the expiring tokens using:
time = 2.hours.ago
UserAuthToken.update_all(rotated_at: time, created_at: time, updated_at: time, seen_at: time)