Messages on OS X stopped working after iOS 7.1 upgrade After upgrading my iPhone 5 to iOS 7.1 the Messages app on my OS X 10.9.2 installation can no longer send messages via iMessage to other iMessage users. Sending messages via other services such as Google Talk is working fine. I have tried signing in and out from my iCloud account in Messages. Also, nothing has changed on the firewall I'm connected through.

Any ideas?
Restart your computer. That solved it for me.