[Permission Required: Administrator, Editor]
If you encounter an XML error when you try to download the screenshot of a published site, you can simply re-publish the site to fix it.
The error message might say "This XML file do not have any style information associated with it."
As you might already know, although those users with permission of author and above can download the screenshot of a site, only editors and above can re-publish the site.
If you need a prior screenshot version for the site, you can let the support team (firstname.lastname@example.org) know to check if other versions are available.