6.7 problems - opening a session

[cakes]

I installed 6.7 a couple of days ago (upgraded from 6.1), and I'm having problems opening two of my session files. They are different versions of the same song - one is a "Save As" of the other. The original session was created by me on this same computer in PT 6.1, and the other was created using Save As in 6.7 after I upgraded.

Here's what happens: about half the time, when I try to open these sessions, PT says "This is not a valid Pro Tools file". Or PT will go through the motions of opening the session (locating audio files, etc.), and then, at the very end, it says "Could not complete your request because file is not a Pro Tools session or OMFI file".

It seems to happen more on the session that was created in 6.1, and it doesn't happen every time. But it's at least 50% of the time. The sessions aren't particularly large (maybe 10 audio tracks, 10 MIDI tracks, a few plugs), and everything in them was recorded on this computer. I'm using a few different inputs of Reason off of Rewire, I have Waves 5.0, uh...the Adaptec 4300 Firewire card...pretty standard Allenstein machine...

Any ideas?

cakes