Multiprocessing broken on CS5, OSX 10.5.8, AE 10.0.1.19

Question

We've been going around and around with our tech staff trying to solve this problem with no avail.  I first posted it here: http://forums.adobe.com/thread/780555?tstart=0 but wanted to make a new thread to see if anyone had a solution.
Our company upgraded to CS5 earlier this month.  Everything went fine for a day or two until BG Renderer stopped working, new aerender instances would not start.  We thought that it had something to do with the script and a few other things, but finally came to the conclusion that it was related to multiprocssing.  If we turn on multiprocessing AE will hard crash.  If we try to launch a render using aerender form a command line (or via BG Renderer), the aerendercore instance never starts.  AE will render fine in single-threaded mode via the GUI.
Deleting the prefs for AE will not rectify the problem.  The only way we can get multiprocessing to work is to create a new user on the machine.  AE will work fine (aerender, BG Renderer and multiprocessing), but after working on a couple projects it will break, usually after 4-5 launches).  We've tried deleting all sort of user prefs both Adobe and Apple with no luck.  Productivity has been radically reduced without the use of background renders.
When trying to run aerender we get this error in the console:
1/31/11 5:05:12 PM [0x0-0x20a20a].com.adobe.aerendercore[4740] After Effects can�t continue: sorry, After Effects has crashed. See http://www.adobe.com/support/products/aftereffects.html for known issues. If you still can�t resolve the issue, please contact Adobe Technical Support (2).
1/31/11 5:05:12 PM com.apple.launchd[107] ([0x0-0x20a20a].com.adobe.aerendercore[4740]) Exited with exit code: 1
I'm hoping someone out there will have had this problem and will have a solution.
OS X 10.5.8
AE 10.0.1.19
8 Core Mac Pro
10 GB Ram (Low I know)

Answer 1

I am pretty sure it is unrelated to any multiprocessing settings and is effected by something else that AE is doing.  We are not really using multiprocessing in-app, but we do use aerender for background rendering quite a bit.  What we've deduced is that both in-app multiprocessing and aerender launch a precess called aerendercore.  One for each processor used.  On the Mac this is program included inside the After Effects package (you can see it if you select "Show Package Contents" when right-clicking on the AE app).  When trying to invoke a render via the aerender command (aerender -project /someproject.aep) you can see this aerendercore process fail to launch by watching the log in Console (the error I posted before).  When we create a new user account (or delete all the prefs out of an existing account) we can get this process to launch, as notices in Activity Monitor.  But something is happening in either the process of closing AE and relaunching or changing settings in app that breaks this aerendercore component.  If we just knew what pref was getting corrupted, that would be something, we could delete it, but it's a little annoying to have to delete all your prefs and have to reset all your OS and AE settings just to get a background render up and running. 
We're at our wits end.  We've tried reinstalling the OS and AE with no luck.  Same symptoms.  If we didn't have to use it (because of project files we are receiving from vendors) we'd just go back to CS4, in which background rendering works fine, but that is not the case. 
The other weird thing is that in CS4 with multiprocessing off, when rendering you can see that AE is using more than one processor (usually between 4-5 by the Activity Monitor, a CPU score of 400-500).  WIth CS5 with multiprocessing off it always uses just one processor (a CPU score of under 100).  So maybe that is a related issue.