AdamM
We had the same issue on a website around 50 gb including emails, it seems that the mail backup and the website backup where running on the same time (mail server is external), and that was causing the file backup to fail.
I am not sure about the reason about the failure but adam checked it and he just removed the .last_backup file , the backups since then run on different times, so it works.
I think there is a bug with this, to be honest because when you do manual backup the backups run at the same time and they dont fail
Edit ***
I just checked and the backup is failing again so this is only a temporary solution there is another underlying issue