Apple Id confusion
Solution 1:
in my experience there are different levels of appleID approval.
for instance, it can work fine for updates but will need to verify with CC info to use the app store etc.
can you go onto a machine with app store or itunes and 'buy' a free app? i suspect it will make you enter more info, or verify a CC security id, or something. after this point, your ID will be blessed for all activities.
Solution 2:
I had the same problem. This happens when you have an older OS X version and you never used your account. Some pages of the appstore will not work with some of the older OS X versions. Apple support gave me two ways to fix this:
Fix 1: Login to the appstore with another apple ID that has already been used by you or a friend. Download and install the update. After you have installed the newest OSX version you will be able to login with your own apple ID and everything works again.
Fix 2: Login to the appstore on another device (Mac, iPhone or iPad) which has a newer OSX/iOS version installed. You can use the device of a friend. After you logged in, your account will be updated. Now you can logout on this device and login on your own device.