Perhaps the available space is not enough when an automated backup is created? I believe a backup uses twice its final size during its creation, because it first puts the files in a folder, then compresses this folder, so it uses more disk space for a brief moment.
Some topics talk about this space issue during backup creation: Reduce local disk space needs by not (redundantly) gzipping backups & Add option to disable backup compression
Not sure this is your issue, though. If you’re confident you have enough space 100% of the time, you can change the Download remote image threshold value to something lower.