I was unable to log in to the Apple App Store while trying to update Xcode on my MBP.
Here's an Apple discussion thread regarding possible solutions.
What ended up working was signing out, clicking "Store -> Create New Account", accepting the Terms and Conditions, then canceling out of new account creation and signing in again.
While deleting the network interface plist, cookies, etc. was educational, it didn't work.
Accepting T&C while not creating a new account so I could use my old one... "It just works"?