I have the same problem but only for 1 website when creating backup. It first worked without problems and then from one day to the next no more. What’s the problem?
The cp command that tries to be executed during the backup process cannot complete the backup process because of the files in public_html that are owened by another user.
This situation can usually occur in files retrieved from backup or manual copying.
How to solve it:
Method
cd /home/exampledomain.com/public_html
chown -R username:username *
Method
You can easily fix it by entering the public_html directory in the File Manager and clicking the Fix Permissions button in the upper right corner.