Keychain Solution
I had the same problem and it was related to my keychain. Got to utilities > Keychain Access. Make sure your key chains are unlocked. When I restarted to see if they were unlocked, they weren't so I just reset my keychain.
I went to preferences and "Reset My Keychain". Restart your computer and make sure the new keychain is unlocked. Then try entering your password in .Mac.
Hope this helps