App store is rejecting my password - please help

I am trying to upgrade to Mountain Lion but the App Store keeps rejecting my password. It is the correct password as I can log in to my account via the internet, and on my phone - I am currently logged in as I am posting this message. I have also tried it a number of times with the same message so it isn't me just making a err each time.

Anyone out there had the same problem that can assist me? Your assistance would be greatly appreciate.