checklink: Proxy Error

W3C Link Checker fails on various sites with the following message:


  Proxy Error

The proxy server received an invalid response from an upstream server.
The proxy server could not handle the request /GET /checklink 
<http://validator.w3.org/checklink>/.

Reason: *Error reading from remote server*

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Apache/2 Server at validator.w3.org Port 80


The site URLs are valid and the their respective hosts are working 
normally.  The home pages come up within a second or two when accessed 
with a variety of Web browsers.