I'm running into the same problem..

And I just figured it out. Wow.. I think Adobe goofed on this one big time.

Anytime you run a droplet, ImageReady writes to a log file. It writes the date and time the files were processed, what droplet was used, and where the destination file was written.. The problem is that this file is owned by the first account to run a droplet. If you have several accounts, the second account will fail because it doesn't have access to write to the log file.

This log file should be in the user's preferences folder, not in the application bundle. I don't know what they were thinking, but they moved it in CS2 (although to a location that is not any better).

To prevent this error message and allow other users to run droplets, you can either delete the log file and let it be recreated by the other user, or change the permissions on the log file so that anyone can write to it.

-------Path to CS file----------
Macintosh HD:Applications:Adobe Photoshop CS:Adobe ImageReady CS.app:Contents:MacOSClassic:ImageReady Batch Log.txt

-------Path to CS2 file-----------
Macintosh HD:Applications:Adobe Photoshop CS2:Adobe ImageReady CS2.app:Contents:Resources:ImageReady Batch Log.txt

I'm submittiing this as a bug as well...