This is an annoying problem that can appear very resiliant to being fixed. This is in part due to a helpful/pesky keychain daemon that caches things that ought not to be cached, even after a user logs out.
1) restart (by any means necessary)
2) login as the affected user
STOP. do not click on stuff. just STOP as soon as you have entered your login username and password and clicked Login.
Did you click on something anyway? If so, return to step 1.
3) it should be saying it can't unlock your keychain. If there's information in your keychain you need (passwords you've forgotten) then you don't want to keep following these instructions. You'll need a bit more personal help than I can provide here in a general post.
4) click to reset keychain to defaults
5) enter the password you want to use to login to the computer. It's okay to use the password you just used. If you enter something different, that is what you will need to use from now on to login to the computer.
6) you may from time to time see an app pop up "asking to use the keychain" since you've changed it. You should be ok to click "always allow" for common sense things. (ical wanting access to your calendar, etc)
That's it. The biggest problem here is users clicking "login anyway", "continue", or "cancel" (varies) during the login errors. That gets the cfprefsd all flustered and it won't leave you alone until you reboot, and you can't fix the problem following ANY of the instructions on the web (short of the Nuclear option) when cfprefsd is doing that. I realize you've fixed the problem with the big hammer already, so this is more of a courtesy post for others searching the web looking for answers to this problem. (we get a lot of traffic looking for help)
