http://www.techradar.com/news/nasty-windows-81-bug-stops-users-logging-onto-their-pc-with-a-microsoft-account

Some folks using Windows 8.1 have been hit by a nasty bug which apparently prevents them from logging on to their PC using a Microsoft account.

The bug was reportedly caused by a recent Microsoft OS update – KB4038792 to be precise, the monthly security rollup of patches for September – and the software giant has acknowledged the problem.