Automated Course Backups not working

Since July 30, 2015 my automated course backups are not working. On the "Reports > Backup" page it says "next backup July 31", which lies in the past.

I assume that's the problem.

Cron is running. I also tried calling admin/cli/cron.php from the command line. The cron runs but no course backups.

I am not receiving emails anymore that backups have been created.

Moodle version: 2.8.7

running on Linux debian

Thanks