Anyone else getting "An unknown error has occurred" signing in to App Store?

Hi all,

I have just acquired a Macbook Air for my wife and after a few bumps, have it up and running except that we simply cannot sign in to the App Store. It's definitely not a userid / password issue (validated via login to app store from iPad/iPhone and set up new apple ID for the wife).

What is happening is simply: when trying to SIGN IN to App Store, I get the error "An unknown error has occurred". Spent 90 minutes on phone with apple support with no luck. I know there have been some server issues due to heavy load with Mountain Lion being so popular but I know folks who have managed to get in to the App Store from their Mac in the time frame I've been 100% unable to.

Any one out there with experience on this? (I've looked at the various threads similar to this but didn't find anything that seemed to help)

Thanks all