Hi. I've been signing in to Windows 10 with a 4 digit PIN number for months.
All of a sudden it's not accepted.
Any rational reason why this would happen?

It's not an issue, I can still sign in with my Microsoft account and change the PIN number, I'm just curious as to why this has happened. If it's a case of PIN expiry or something, then I would expect some form of notice.

(Please, nobody ask me if I'm inputting the right number :) ).