Could Not Log In to iMessage–Resetting PRAM fixed it
All of a sudden on my MacBook Pro I could not log in to iMessage. It had an error saying it couldn’t find the right settings or some sort. I tried resetting passwords. Logging in on various versions of the .mac, .me and .icloud logins I have. Nothing worked.
Then I tried resetting the PRAM on the Mac and it works fine now. A link to how to do this is below, but essentially you turn off the machine and before you hit the power button, hold down the OPTION + COMMAND + P + R keys all at the same time. (At least before the white screen appears, and then let go. The screen will go black. When it boots, you may have to hit the power button again, then try to log in to iMessages and at least for me, and others, it worked.
Not sure what’s afoot. There was some speculation that this was due to iOS 7 beta conflicts or a new software update for Mountain Lion, but I didn’t have one available and I’m not using iOS 7 beta on any of my devices.
Hope this works for you.