Sorry for wasting your time, gentlemen, but I solved the problem by simply trying my wife's Microsoft Account password again, myself, on her login screen. It worked. Which is a good thing, because...

Thanks for your reply, simrick. I don't know how to tell whether her login screen shows her local account or the MS account; it looks the same as it used to, though, and there appears to be only one...

My wife used a local password to log into Win 10, and got a Microsoft account, specifying another password. The next time she booted up, neither her old nor new password was accepted. We don't have a...