CAUTION!
I just tried the second workaround that Durin spelled out in the previous message, opening the sesx file in Apple TextEdit and changing two instances of the word ‘mono’ to ‘stereo’. I then saved the file in TextEdit and tried to reopen it in Audition. I got the following fatal error message:
/Users/Flawn/Desktop/Burma FOR FLAWN/A SEG_01_CopiedFiles/A SEG_FW_Mix copy.sesx
Error: The file is either corrupt or using an unsupported format
XML FATAL ERROR: (line: 5, col: 66) [ /Users/Flawn/Desktop/Burma FOR FLAWN/A SEG_01_CopiedFiles/A SEG_FW_Mix copy.sesx] attribute value expected
Now I cannot open the session. So, at the very least, proceed with caution, and try the workaround on a copy of the file before you expose yourself to losing work!