I have the same questions as Steve. The Guitar Tracks version is a biggie.
It used to be that all of the Cakewalk software shared the same basic format, and forward/reverse project sharing was generally pretty easy.
At one point, someone sent me a Sonar 8 Producer bundle file, and it was amazing to watch it open in SHS6. Missing plug-ins were clearly defined, and it was fairly easy to make adjustments.
Enter the X series. There is a lot of information in an X1 project file that the earlier versions cannot digest(screen sets, for example).
Whenever I open a project from SHS6 or Studio 8 in X1 or X3, I get a warning that the saved project will no longer be able to open in the older versions. I believe you may be running into the same thing.
However, you can exchange audio tracks as wav or MP3 files. On the whole, I think this is a better way to do it anyway.
You don't really need to send the whole project back and forth, just the individual track(s). This how many of us do collaborative work.
Since you have the more sophisticated software (ie; more options for processing the mix), we'll assume you are the master for this project.
Export what you have so far to an MP3 file. Send it to your friend. He now has a reference track to work with and do his stuff.
He exports his track to a wav file, and sends it back to you. You import his wav audio file to your master project.
Make sense?