Issue Server backup are empty

[Henry C. Lavau]

Hi,
My Plesk:
Plesk Obsidian Web Pro Edition
Version 18.0.21 Update #5

Disk size:
root@xxx:~# df /tmp -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda1 79G 36G 40G 47% /

When I ask for Download from the Backup Manager, I have
Backup size 3.60 GB (this addition) + 153 MB (previous additions) + 7.59 GB (initial backup) = 11.3 GB

I just get on my computer is a .xml.tar file of 1.53 Kb containing just a directory of size 0

How can I donwload a full server backup?