Scheduled Backup failed every time

Hello,

I have hosted a wordpress website version 5.7.2. All plugin are uptodate.
It is installed on an ubuntu server 20.04.2 x64.

I use your BackupGuard Plugin, what is working fine, when I start a full-backup manually with then blue button “Backup”. Then everything is fine. Size of the full-backup from 2021-05-17 07:23:00 is 6631,61 MB.

But when the full-backup starts automatically with scheduled backup, then it always failed.Backup-size is empty. Date from failed backup is 2021-05-18 05:10:00

Can you please help me to fix this?
Is there a Log-file to check the last failed backup?
What information do you need?

Thank you very much.

best regards,
Alexander