Hi Daniel,
In both cases the backup failed in its last stage which is a cleanup stage.
In that stage empty files, that were created by the mapreduce library (for kinds with a small amount of entities), are being removed.
The failure is triggered when there are more than 500 files to be deleted and is going to be addressed soon.
I can either cleanup your failed backups or change them to be marked as complete (leaving the empty files in place).
Also, as a workaround to prevent this kind of failure, until a fixed is available, I suggest you backup 15 kinds or less at a time.
Arie.