I just found out while testing in a new environment with a copy of our production site that the automated backup just stops when a corrupted backup is found in the backup directory. This means that the automated backup process is out of sync at that moment. All the courses that are not yet read by the automated backup script remain with the nextstarttime of the last (failed) run. These courses will never be backupped anymore. My backup broke after having read a third part of all courses in mdl_backup_courses. I deleted the corrupted backup, made a new (manual) backup of this course to be sure if I was able to backup this course and uploaded the backup to the map with all the (automated) backups. Tonight I'll see if there's another course in my map that'll cause the automated backup to stop.
This was the error I got at the point the automated backup was prematurely ended (and it said it wasn't possible to read the archive):
!!! Fout bij het verwerken van het archiefbestand !!!
To be sure that all courses will be read by the automated backup script I updated the nextstarttime for all rows in mdl_backup_courses with the date of the next scheduled automated backup run (tonight):
mysql statement: update mdl_backup_courses set nextstarttime = unix_timestamp('2019-03-30 00:00:00');