Apple's yearly developer conference should see iOS 8, OS X 10.10, and likely some hardware.
Some Users Unable to Log In to App Store Due to 'FATAL' Error [Updated]
The problem, which seems to have originated last night, is ongoing according to a thread on Apple's online Support Communities and multiple users on MacRumors' forums. The error occurs on both mobile devices and Macs and according to user reports, it is affecting the App Store, the Mac App Store, the iTunes Store, and the iBookstore.
Apple’s System Status page has thus far not reported any issues, but according to one MacRumors reader who contacted AppleCare, there have been ongoing problems with Apple ID logins. Yesterday, some users were also getting a different error: "This Apple ID cannot be used for the iTunes Store. Please enter another Apple ID."
The multipage thread on Apple’s support forums suggests that the issue is fairly widespread and there appears to be no fix available at this time. Multiple users have reported the issue to Apple and the company appears to be aware of the problem.
Update: A number of users are reporting that the issue has been fixed and their accounts are now working normally again.