Apple rightfully has a reputation of having its products "just work". And 99% of the time that adage holds true. Until it doesn't. When things do go wrong, Apple's error messages are cryptic, at times offering no obvious clue as to how to remedy them. One such error is the "Authentication Error" message when trying to log into iMessage and FaceTime on your Mac, even if you've successfully logged into iCloud.

The problem

For a non-obvious reason, my previously properly running Mac could no longer log into iMessage or FaceTime in spite of being successfully logged into iCloud. Notes synced, Documents synced, Photo stream working, continuity functional, etc.

Trying to log into iMessage triggers my two-factor authentication mechanism as expected. But when prompted for my iCloud username and password, and after a long timeout period, I simply get an "Authentication Error".

The potential fix

The fix involves getting Apple's servers to re-initialize your mac hardware to be allowed to access your iCloud services.

Log out of iCloud on the Mac

  1. Open System Preferences.
  2. Select iCloud.
  3. Select Sign Out.
  4. Select Keep a Copy.
  5. Allow your files to download locally.

  6. Select Keep on This Mac to keep saved credentials locally.
  7. Restart you Mac.

Log into your iCloud.com web account

  1. Open Safari.
  2. Navigate to iCloud.com.
  3. Log in to your iCloud account.
  4. Select Settings.
  5. Select your problematic Mac.
  6. Click the X to remove the Mac.

  7. Affirm the deletion.
  8. Click Done

Log back into iCloud on your Mac

  1. Open System Preferences.
  2. Select iCloud.
  3. Enter your Apple ID.
  4. Click Next.
  5. Enter your Mac User Name and Password.
  6. Select OK.
  7. Click Next.
  8. Click Allow.

Log into iMessage

  1. Start iMessage.
  2. Enter your iCloud credentials.
  3. Click Next.
  4. Done!

Final comments

This is but one solution (of the myriad of suggestions found online) that worked for me. Let us know in the comments if this fix helped you too!

macOS High Sierra

Main

macOS Sierra