This appears to be the issue where nginx has failures and the system requires a restart for your scheduled backups to begin working again. Note that manual backups will likely run to completely but the scheduled backups will continue failing until you reboot. I have seen this same thing on my systems when I get the failed backups error. Try a reboot of your phone system and let the schedule run again. This has supposedly been fixed on 15.5 SP2. I have not yet deployed it yet as I wait for it to "bake" a bit before deploying into production.
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