by Alain Raap.
Ken, we could shake hands I found the same resolution/strategy like you did.
As we have several large courses (> 2,5GB) I found out that one of the courses took 7 hours to complete (with the cli script via a cron bash script)! And a few others that caused other errors where the automated backup was broken .
I tested your truncate solution of the table mdl_backup_courses and that is also a way to get it working again. But that was not on our production site.
I am looking to implement a solution as you propose, just to exclude the courses that cause all these troubles.
I hope the next version of Moodle (3.5.x) will have a better implementation of the automated backup process