Adobe Audition CS6: The file is either corrupt or using an unsupported format

I have an Audition CS6 multitrack session (sesx) file which is refusing to open. The error message says "The file is either corrupt or using an unsupported format". The strange thing is that I saved three different sesx files at various points while I was working on the project (to ensure I had a backup and could revert changes if need be) and I get the same error message when I try to open any of them.

Luckily I exported the finished project as wav before I closed Audition but I'd nevertheless like to find out what the problem is as it's quite worrying to think that I could have lost days worth of work even when I regularly saved backup copies of the session.

Any help in diagnosing the problem would be much appreciated.