I've had this problem with my Aunt's 2010 iMac; she forgot the master password and I performed a reset using the install disk. However, since then she has been plagued by the same issue detailed above, repeated requests for password. I finally had a chance to go to the Apple Genius bar having exhausted all discussion forums to no avail and was given an as yet unproven solution. The guy says he gets this problem all the time and he was pretty confident it would work.
- Close all programs except the Finder and go to Home/Library (may also be Username/Library)
- Find the Keychains folder in here; delete it
- Log out / Restart immediately
- Keychain will recreate the folder and you should be good to go
I will update this post on attempting this fix; it sounds logical though. Anyone who has success, feel free to chime in. I should also point out that this will obviously wipe any application/web passwords you already have saved but I don't think that should be an issue for most people.
Update: It worked, so far so good! Can't believe the solution was so simple!