Messages.app - unable to sign in with Apple ID/OS X 10.8
I'm unable to sign in to iMessage with my Apple ID/password on Mac.
Hardware:
2 x 2.26 GHz quad-core Intel Xeon Mac Pro
32 GB RAM
Network connection via wired Ethernet - confirmed operational
OS:
OS X 10.8
Procedure:
(1) Launch Messages.app under OS X 10.8
(2) Select Preferences > Accounts
(3) Note iMessage (Inactive) in accounts list
(4) Enter Apple ID and password.
(5) Click Sign In
(6) Note indeterminate activity indicator (spinner) followed by alert sheet: "Could not sign in to iMessage. Could not sign in. Please check your network connection and try again."
Attempted already:
(1) Changed router DNS settings to 8.8.8.8 and 8.8.4.4 for primary and secondary respectively.
(2) Deleted every plist remotely related to Messages, IMAgent, iChat.
(3) Verified iMessage works with same Apple ID on iPhone 4 and iPad gen 3.
(4) Restarted computer after (2)
Messages-OTHER, OS X Mountain Lion