On the heels of reports of sporadic installation and authentication issues with iOS 9.3 by owners of the 2011-era iPad 2, new reports are claiming that an unknown but possibly small percentage of Mac owners are seeing authentication errors when trying to use FaceTime and iMessage under the new OS X 10.11.4 El Capitan. The problem, which appears to be the result of a geographic node gone wrong rather than a widespread problem, does not affect other iCloud services, and did not appear during beta testing.
