I have had this error many times over the years. It is an intrinsic bug within Moodle on its backups, and I am surprised that Moodle HQ has not given it more priority. But I guess the old saying is true, that it ain't in my backyard, so?...
Anyway, I have traced this issue to a backup that hangs on a particular part of my site. It gets to an error state, and then there is no error handling in Moodle. So, it just sits there, hoping that the error will go away, and of course it never does, so the backup report shows that there was an error, and no explanation.
What I finally did was did some manual backups, eliminating one thing at a time until a complete backup finally happened. That helped me narrow down what the issue was. For me, it was the inclusion of Activities in the backup.
But I wasn't sure which activity it was, so I selected them one by one until I found out which activities. Turns out for me, that it was an assignment in which I suppose there were too many student submissions and Moodle choked on it. I eliminated that assignment, and it started backing up again.
I will add that you should go into the automated backups and change the time that backups should occur, then go to "your site name and domain/admin/cron.php" and run the cron.php script. This will reset the time things should run.
Then go to the reports and see if a backup happened. If so, all will be green. If not, then experiment with what should be backed up in the automated backup settings until you get a good backup, even if some things are missing. The goal is to reset the timer in automated backups so you get all good backups of all your courses.
I am curious to know if this works for you. Please let us all know.