Messages.app can't connect to AIM/GChat/FB Jabber, AIM.app works
Since updating to 10.8 on my 2011 MBP, I've had a ton of trouble with Messages.app . It seems to work consistently from home on a Frontier DSL behind a 2003-era Airport Extreme. However on my business network (Comcast, behind Sonicwall TZ170 & Windows 2003 SBS server), connection is inconsistent at best. This morning, I'd left my MBP logged in & sleeping from home, and plugged in at work. Messages remained logged in to AIM, GChat & Facebook via Jabber. However, after a restart, those services will not reconnect.
Messages lost the connection to the AIM account “xxxx”. A network error occurred.
Messages lost the connection to the Google Talk account “xxxxx”. A network error occurred.
Following other tips on here, I've dumped a variety of .plist files, removed my accounts & added again, set preferences to use separate Buddy lists, and checked/unchecked/checked again the use SSL options. This started to feel like a DNS issue. But, I'm running XPsp3 via VirtualBox, and that computer's AIM & Yahoo connections on Trillian work just fine. Then I downloaded & installed the official AIM.app, and it also connected without issue. I can ping each of those hosts from the command line.
Before upgrading, I had been using iChat on 10.4/10.5/10.6/10.7 with those account without problem. Strangely, Bonjour rarely connected under those configurations but does now every time. iMessages work properly.
MacBook Pro, OS X Mountain Lion (10.8.4)