Scheduled backups are failing suddenly (but work when I do them manually)

My automated backups fail due to a free space error, but the disks are far from full. When I invoke a manual backup to the same storage, it works fine. Here are the emailed logs from both situations. The backups are heading to a PBS instance running as an unRAID container which has an external USB 3.0 enclosure connected to it. If you have any ideas, I'd be thankful for some input! Thanks!

Excerpt from failing automatic backup log:

vzdump 104 107 100 101 112 113 201 106 115 116 120 103 102...

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