This error happens in Plesk when you try to make a backup from the web interface.
The cause is that Plesk can´t write in the temporal folder specified to storage backups because the owner is root:
ll /var/lib/psa/dumps/
drwx------ 9 root root 4096 Apr 1 14:21 tmp
To fix this change the ownership of the folder:
chown psaadm.psaadm /var/lib/psa/dumps/tmp
Now you can do backups in Plesk.