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Your checker can't check my site, but when I click on the link from the page that gives me the error message, the browser goes straight to the error page
The browser goes to what error page? Anyway, this is a problem with the DNS setup of your site, not the validator. I tested from two hosts in the U.S., one in Japan, two in Finland and one in France, and only one of the U.S. hosts was able to resolve the www.comune.mercato-san-severino.sa.it hostname. Try contacting whoever administers the DNS for your site. An example online service for debugging DNS problems is http://www.squish.net/dnscheck/