What is the meaning of this "scanning error" ? 

Scanning Errors

If our scanner encounters an error, it may decide to remove the page’s eligibility for automatic scanning. You will know that this is the case if you see an “X” next to the page in the Pages tab. An error may be one of the following.

• Invalid status code.
  • This happens if the page returns a 404, or any other status code our scanner deems unfit for future scanning.
• Aborted, possibly due to unsupported content.
  • Something about the page's content type prevented proper scanning. This will often happen if you try to scan a PDF or CSV, for example.
• Too many redirects.
  • Occurs if a redirect loop is detected.
• Domain not resolved.
  • If your domain is not properly resolved to an IP address.
• Different hostname.
  • To ensure that we aren’t scanning external websites, we verify that the hostname of your website is the same page we land on. This means that we only scan pages that live on your website. It’s important to know that the hostname www.example.com is considered a different website than example.com. Make sure that your primary domain is set up properly.

For more information on common site scanning issues, visit our page on troubleshooting.