It makes perfect sense, in that we've heard a fair amount of similar posts along this same line.
It makes absolutely no sense what so ever as to why this is happening to so many people.
At the sign-in screen look around for any icons or links that may indicate something related to or mentioning
"Language or Region" or "I'm not" or "Not Me" or anything like that.
Additionally use the OSK (On Screen Keyboard) from the accessibility icon on the sign-in screen and confirm
1. That you are typing the user name accurately
2. Combine the OSK with the "Eye" spy icon in the password text box to confirm that you are entering the password for that account exactly as you intend (case sensitive).
We've heard mention that an update has reset the language and keyboard layout to something other than what you expect to be default (English UK or English US)
We've also heard mention that an update has resulted in a keyboard driver problem, especially on laptops resulting in some key strikes not producing the proper alphanumeric character or symbol.
