Cannot validate xml sitemap

[merrick777]

What does this error mean? What do I do if I continuously get this? This is what I get when I enter the url of my sitemap.xml into the sitemap validator tool.

Proxy Error

The proxy server received an invalid response from an upstream server.
The proxy server could not handle the request GET /2001/03/webdata/xsv.

Reason: Error reading from remote server

[XML-Sitemaps Support]

Hello,

official w3c validator tool is currently offilne, so it is not functioning at this moment.