I just updated my MacBook Pro (Early 2011) to OS X Yosemite, now iMessages is no longer working. I turned it off/on on both my computer and my iPhone, reset my Keychain, and a bunch of other stuff. Nothing worked.
How can I fix this ?
After an hour long call with Apple, I was finally told to reset my default Keychain, and after a reboot, that worked. We diagnosed this as the issue because every time I rebooted, iCloud/FaceTime/Messages would ask for my password again, even though it had supposedly saved it.
This also came right after I had just reset the default Keychain to solve not being able to connect to my wireless network.
You'll just have to wait till iOS 8.1
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