zipimport IOError reading Modules.zip in Mac standalone app

[Bob Swerdlow]

We have some users of our application getting error messages like:
    IOError: zipimport: can not open file 
/Volumes/MyApp/MyApp.app/Contents/Resources/Modules.zip
This only happens on our Mac version - the Windows version seems fine.

Our build is still using bundlebuilder, which creates Modules.zip.  I want 
to upgrade to py2app, but have not yet done so (we are using py2exe for the 
Windows version).

Only a few (7) of our users have had these IOError problems other users are 
running fine.  The errors they were all either:
    in shelve.__init__
    in encodings/__init__.py
    or in the import to one of our own modules.

When I look into the Modules.zip file, I see
    shelve.pyo
    encodings/__init__.pyo
    and our own module's pyo file

So, I don't understand why we get these zipimport IOErrors.

The application launches more than one process that uses the embedded python 
(we started using the new subprocess module in this release).  Could that be 
a problem?  Could it be that more than one process is trying to read 
Modules.zip at the same time?  If so, what do we do - I don't want to have 
to include a separate version of the code for each subprocess.

Any other suggestion?  I need this fixed ASAP.

Many thanks,
Bob