"A Severe Error Was Encountered (Code 0000000B)" in final mixdown of multitrack session.

I'm just finishing up a podcast, which has six tracks in Audition 13.0.13.46.  I'm editing on an old iMac -- mid-2011 which has topped out at High Sierra Version 10.13.6.  This has caused numerous crashes, but uintil I can afford a new computer, this is the best I can do. 

I did the mix last night, and have now attempted to do a Mixdown to a New File for export and publishing.   The process aborts with this ominous red alert:  

I have been backing up the session with all files on an external drive, so can backtrack if necessary (this did not happen on previous mixdowns).  But obviously, it would help to know what can trigger this scary alert.

Any thoughts?

THANK YOU!  

Incidentally, we are planning a future episode on how technology is increasingly "incompatible" with Baby Boomers like me, and we're seeking input from experts and hapless laypeople as well.  Please let me know at ron@ourgenpod.com if you have any thoughts on that subject.  (Forums like this one are going to get lavish accolades. )