CAS System Unable to Initialize ClientHandler

This error message usually signifies an issue with the Client Access Security (CAS) system responsible for authenticating and granting access to the game. Please try these steps to see if it helps you: 

Windows

1. Enable Secondary Logon
2. Disable read-only settings for WoW folder
3. Delete the "Indices" folder inside "World of Warcraft\data" folder
4. Scan and Repair the game
5. Create new administrator user account

Mac

1. Repair Shared Folder permissions
2. Repair Disk Permissions
3. Repair Application Support Folder permissions
4. Scan and Repair the game
5. Create new administrator user account