The backup restoration completes with the warning under Tools & Settings > Backup Manager
Error log: Warning: Restore hosting "example.com"
Can not deploy content of domain example.com
Unable to extract files from /var/lib/psa/dumps/domains/example.com/backup_userdata_1811011003.tgz to /var/www/vhosts/example.com:
/bin/tar: httpdocs/index.html: Not found in archive
/var/www/vhosts/example.com/httpdocs/index.html file is missing in the backup archive.
This happens, when during the backup creation some files are removed. Thus, they exist in the configuration file but missing in the backup as itself.
Restore the required files from another backup.