I can reproduce this to 100% on two differently configured computers.
On Mac OS 10.7.1 with Pro Tools 9.0.5 installed:
- It works fine as long as iChat hasn't been started.
- When I start iChat (even when it has never been started before) and then try to start Pro Tools it crashes after Loading the Plugins.
- Exiting iChat doesn't make Pro Tools work again - I have to fully reboot before it starts again.
Crashlog is here:
http://pastebin.com/ccaWJRvt
Update 1 (29. Sep 2011 17:48 GMT+1)
A new workaround has come up. When Pro Tools no longer starts up you can go to Audio MIDI Setup (via Spotlight), choose Pro Tools Aggregate I/O and on the right side deselect all built-in sound sources. Then Pro Tools will start up fine and seems to fully work as long as you change the Setup -> Playback Engine Setting in Pro Tools to a valid sound card.