Mac users who have updated to OS X 10.11.4 El Capitan found that they are unable to log in to FaceTime and iMessage, leading them to flood the Apple forums with complaints.
Most of the user-submitted reports involve the error messages of "Could not sign in to iMessage" and "An error occurred during authentication" popping up. Aside from those, other users said that everything seemed to be perfectly fine as the iMessage window appears to connect to Apple servers, but it doesn't actually do so without informing them with a warning message.
— Ase Deliri (@AseDeliri) March 23, 2016
One user who goes by the name of ivanpasic encountered the issue after a clean installation of OS X 10.11.4 on a MacBook Pro.
"I did a fresh install of OS X 10.11.4 on my MacBook Pro. Now I cannot log [in to] iMessage or Face Time. I contacted Apple and was told there is an issue with OS X 10.11.14," he says.
Other users have tried various ways to fix the problem, including updating, restarting and turning the iCloud off and on. The list goes on with doing a fresh reinstallation, turning the firewall off and turning the two-step verification off and on.
Another user who bears the moniker bevnetcraven discovered that going from OS X Yosemite to OS X 10.11.4 El Capitan doesn't introduce the login issue. In other words, the bug is usually present on devices that underwent a clean installation of the update.
To make matters more complicated, some users have tried to downgrade to OS X 10.11.3 El Capitan but to no avail. That means the problem doesn't necessarily lie in the update as the issue could be on the side of Apple's servers. However, this comes off as odd, as not every user is experiencing the bug.
To boil things down, a solution has yet to come, but based on the user comments and Apple's System Status page, the company is aware of the issue.