The title says most of it. My mother changed her password through the Accounts pane of System Preferences to a lengthy one (17 characters) about three weeks ago. Then, the next day, she left for Spain, and she came back today. I'm pretty sure she didn't log out between the time she changed her password and the time she shut down her computer on the day she left.
Now, when she logs in, the system shows a dialog box that says it can't unlock her login keychain, and suggests that she either updates the password or create a new login keychain. I tried with her old password, a variation of her old password, her current password and a few variations of her current password and nothing does it. My only conclusion is that somehow, something changed her session password to something that is neither her old password nor her new password. The question now is, what could have done that, and what can it be?