Yahoo does have frequent outages, which can cause the same message...
Open Keychain Access, search for yahoo, remove those entries & carefully put them in next time it asks.
Mac OS X 10.4 Help, I forgot a password in my Keychain
Resetting your keychain in Mac OS X...
If Keychain First Aid finds an issue that it cannot repair, or if you do not know your keychain password, you may need to reset your keychain.