You sure have had issues with autobackups. Other than moodle version might be a good idea to share some more info about setup ... overall ... any tech that would/could be related to auto issues. Setup in a VM? etc.
Don't have an answer ... just suggestions/things to check ... maybe do?
in scheduled task there are 2 related to auto backups (both in 3.7 and 3.9):
One is: Automated backups \core\task\automated_backup_task
But you say you are doing auto backups outside of Moodle.
Your message clip did show 'check logs' ... did you?
IF so, what did logs show/say?
The other:
Clean backup tables and logs \core\task\backup_cleanup_task
There is also:
Delete stale temp files \core\task\file_temp_cleanup_task
As you know scheduled task have their own set ups for frequency/time to run.
There is no task for scheduling notifications related to autobackups.
In addition, you are the Moodle Admin but not maybe the server admin/op.
Servers have other things to do so there are other cron jobs scheduled
to do whatever they do. Servers also have internal checkers ... example:
checking apache service it sees its not responding so it is set to restart
the service. Could the server OP have done something to upset auto inadvertently?
Am wondering if there is some conflict that messes with the cron job for auto backups.
One would have to compare *every* auto message received since the site was upgraded from 3.7.x to 3.9.x to see history of this.
Since your site has had a history of issues with auto backups, would think
that immediately after an upgrade that would be the first thing checked.
Had to truncate all the tables related to auto backups once to start over cause in logs/schedules I saw a couple of courses that had next run date a year into the future.
Yes, truncating all the tables means starting over .. but you are backing up
every night anyway. I hope to a directory outside of the sea or files in
moodledata/filedir/
Maybe you could create a new location for the 1st run after truncating tables and then do a comparison of old location vs new location backups.
'SoS', Ken