The dashboard could sign you straight back out again

After signing in successfully, the dashboard could decide you were not signed in, send you back to sign in, and repeat — refreshing over and over until the browser gave up. Your sign-in was working correctly every time; the dashboard was checking for a marker that used to be written by the old sign-in page, which was retired earlier the same day, so it never found one and threw a valid session away. It now asks our servers whether you are signed in rather than guessing from what it can see in the browser, which is the only thing that actually knows. It also stops after a few failed attempts and explains what happened instead of looping.