I just upgraded to Mavericks. Now, every time I wake it up from the lock screen or standby and proceed to type in my password it rejects it.
To get back in I have to select "switch user", then type in my username and the (same) password. Afterwards it requires me to set a new password and only then it lets me in.
This is, as you might imagine, very annoying. How can I get rid of this new security requirement?