404 page not found error even though the page exists
I finally resolved this after a lot of hair-pulling and gnashing of teeth by adding a URL alias from node/133 to node/1. Google Webtools now validates, and do does w3 link validator.

I discovered the solution by looking in the error logs:
 - admin/reports/page-not-found

I noticed that the specific page throwing the error was node/1 . Neither google Webmaster Tools, nor the validator helped as they reported only the 404 on the main domain name.