Yesterday, if you performed a factory reset of a Windows Phone 8.1 device, or rolled a Windows 10 Mobile handset back to the old OS, you probably found that you were unable to sign into a Microsoft account. In fact, you wouldn't even be able to turn off Reset Protection without going online and finding a code.
We've still been unable to get an official statement from Microsoft regarding why this happened (inclement weather in Redmond has made communications a bit rough, it would seem), but the issue appears to have been resolved. Both turning off Reset Protection and signing into the device are working on all of the handsets that we've tested, including the Lumia 640 that we used yesterday.
Again, it's not clear why this happened at all, or why it didn't affect Windows 10 Mobile devices. Part of the problem was that if you're on Windows Phone 8.1, you can't get back to Windows 10 without first downloading an app from the Windows Store, for which you'd have to be signed into a Microsoft account.
Luckily, everything seems to be fine now, and when we get more information from Microsoft, we'll let you know.