I may be shooting in the dark here, but....
Sounds like some kind of file corruption. I go to File>Save As, then check the box for Audio Files, and(this is the crucial step) check the box for Main Playlist Only and save this to your recording drive(and NOT inside the original session folder). If this is successful(the copied session saves), close the original and open the copy version. If the copy runs, saves and closes, then you have a good version, and you can be pretty certain some unused audio file(s) is corrupt in the original session.
If you are still curious about the ?(and I am too), open the original session and select for all tracks to show Playlists, and look for an audio file with a ? or maybe some undecipherable file name(if the session is huge, this will use up some time, which maybe you don't feel like wasting
). Also, open the Clips bin and scroll up/down to see if any clips are off-line(and maybe remove/delete if you find any).