In my Google Webmaster Tools, I received an error which I am not understanding:
When we tested a sample of the URLs from your Sitemap, we found that some of the URLs were unreachable. Please check your webserver for possible misconfiguration, as these errors may be caused by a server error (such as a 5xx error) or a network error between Googlebot and your server. All reachable URLs will still be submitted.
I received this warning for a URL that is currently working perfectly. Can you please tell me if this a serious warning, or what it means?