Backups empty when downloaded

Hello,
I'm encountering a bit of a weird incident. I'm using "NEW Backup (Beta)", usually it works correctly, but in this server it generates files and folders on my local NAS, but the files are empty (example.tar.gz have 0).
I download the files using ftp, and i have not any incident with other installations CWP and no more too with other control panels.
In the server they have the correct MB, and i can generate a backup to keep it in the server, and it works.
Does anyone know why this can happen?
Thanks for your attention, and sorry for my English.

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