If you're someone that manages your fonts more than a normal user, you may have run into this issue with the built-in Messages, or iMessage, app on OS X. My users couldn't sign in or access any of the app features. The reason behind it was really simple. The AppleGothic font that normally comes on the system is required for the Messages app to work correctly. Make sure you have it activated if you're using a font manager, such as Universal Type Client. Once you have it active, open Messages and you should be able to use it like you would've expected.
Credit: https://discussions.apple.com/thread/6037163?tstart=0
