Backup domain fails

Hello and happy new year !

I using the latest virtualmin gpl with CentOS 5.1 and the default install and when I create a backup domain I have this error:

What can be happening ? The output directory have write rigths for virtualmin.

Thanks in advance.

Roberto

Starting backup of 1 domains to local file /root/backup/gualeguaychu ..

Creating backup for virtual server gualeguaychu.gov.ar ..

  Copying virtual server configuration ..
  .. done

  Creating TAR file of home directory ..
  .. done

  Backing up Cron jobs ..
  .. none defined.

  Saving mail aliases ..
  .. done

  Saving mail and FTP users ..
  .. done

  Backing up mail and FTP user Cron jobs ..
  .. none to backup

  Copying records in DNS domain ..
  .. done

  Copying Apache virtual host configuration ..
  .. done

  Copying Webalizer configuration files ..
  .. done

  Copying Logrotate configuration ..
  .. done

  Backing up Webmin ACL files ..
  .. tar failed :

  Backup failed! See the progress output above for the reason why.