im a windows guy and dont know too much about mac, this is my wifes mac,
she has had the same user name and password for years, and this just started today,
i get to the login screen enter user name and password and get the "shaking box" like the password is incorrect,
i unplugged our wireless router so the computer would NOT connect to the internet, started up the computer and the password WORKED!
plugged in the router, restarted the computer, and same problem, the "shaking box"
basically the password will work when the computer is NOT connected to the internet, but does NOT work when the computer IS connected to the internet,
i suppose the easy fix is to disconnect from the internet before shut down and reconnect manually ever time she starts it up, but that a pain in the.....