Stuck at login in App Store
Grr - the store is having a rough week.

I would do these three things:


Make a new admin user account and log out of the broken "MAS account" to get a clean state.
Run the app store on the new user account. You don't need to sign in for security updates, so this will test that Apple's side of things is working and/or your core system is working.
If that doesn't work, open terminal and run these two exact commands:

&nbsp;

softwareupdate --reset-ignored

softwareupdate --install --all  



You can take a picture of that screen and then call AppleCare or open a web support ticket if you can't get that to work. They should cover that for free since it's security related as well as they generally support anyone's Apple ID that would be used to spend money on the store without charging them for that incident of support. The core updates should work even when your account or the Apple servers for authentication are overloaded. When that happens you get errors like you mentioned and Apple's support pages rarely log an error until everyone is locked out. (i.e. thousands of people could have problems and it's not a big issue when millions of accounts still work).


http://apple.com/support/systemstatus


I use that "contact us" link from the system status page to initiate my support requests for cloud and Apple ID issues :-)