Hi,
Check and install if required AppleGoth.ttf
This font has been found to be missing in Hard Drive/Library/Fonts and is needed during the "registration" process of the iMessages account (which it goes through each time the app is relaunched).
iMessages accounts (Adding the Apple ID you want to use) can only be done in the Messages app.
They are not added in Internet Accounts. Nor are they part of iCloud (it does seem that Mavericks/Messages 8 want the ID to be linked to iCloud)
Messages will use AIM, Jabber as well as Yahoo and Bonjour and iMessages.
AIM and Jabber can be added in Internet Accounts.
Apple IDs issued by Apple (@mac.com, @me.com and @icloud.com) can be used for AIM valid Screen Names as well as the iMessages Apple ID.
Jabber includes Google and Facebook.
There are now limits on using Older @me.com and @mac.com names on older versions of iChat
OS X: Using me.com or mac.com IDs with AIM in iChat requires OS X v10.7.2 or later
7:29 pm Friday; July 11, 2014
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad