"session created with invalid configuration of Pro Tools"

[aconk]

I have a PowerBook G4 with OS 10.3.9 running Pro Tools LE 6.8.1 with an Mbox 2, and I am trying to open a session created with a Mac mini (OS 10.4) running Pro Tools 7.3 M-Powered with a Firewire 1814 interface (so that I can do overdubs at my leisure). Of course, we tried converting the session format from .ptf to .pts by going to "save copy in" and selecting the PT 5.1-6.9 format. Additionally, we lowered the session's sample rate from 96 to 48 (the max rate for PT 6.8). None of this seemed to work - we still received this error message: "Could not open because the session was created with an invalid configuration of Pro Tools". Is there anything else that can be done in this situation? Does the session have to be created differently from the beginning for this kind of backward compatibility to work?