Messages having issues logging on recently on Yosemite? Just within the last day or two I have had Messages go out on me.  It happened with 10.10 and then the same is occurring with 10.10.1.  I see that others around the web are experiencing the same or a similar issue from the way things are phrased, yet no one seems to have a clear answer.  One individual seems to have gotten some help by talking with Apple and having them look at their Apple ID account and changing something on Apple's end... but after being on hold for an hour and them telling me that they're just going to have to have someone call me back, I thought I'd start asking for some additional input here:

The first indicator of the problem is simply this message in iMessage whilst trying to sign in:



I then discovered the following messages in my system.log associated with each time I attempted to login via iMessage:

com.apple.xpc.launchd[1] (com.apple.imfoundation.IMRemoteURLConnectionAgent): The _DirtyJetsamMemoryLimit key is not available on this platform.
com.apple.SecurityServer[56]: Session 100180 created
com.apple.SecurityServer[56]: Killing auth hosts
com.apple.SecurityServer[56]: Session 100178 destroyed
logind[71]: -[SessionManager getClient:withRole:inAuditSession:]:241: ERROR: No session dictionary for audit session 100180
logind[71]: _SMGetSessionAgent:73: ERROR: __SMGetClientForAuditSessionAgent failed 2
IMRemoteURLConnectionAgent[6887]: SACShieldWindowShowing:925: ERROR: NULL response


Anyone have an idea on what may resolve this?
In this order, try this:

Sign out of iMessage on all devices (if you want to be lazy, just the computer)

Remove iCloud from the computer (in the Accounts tab -- don't worry, we'll be putting it right back on)

Restart the machine

Repeat in reverse (add your iCloud account back, and then sign into iMessage).

That's basically what AppleCare would have you do.