Error (5XX) URL in XML Sitemaps
This means that the URL in question returns an HTTP 'Error' status (5XX), yet is included in an XML Sitemap.
Why is this important?
Your XML Sitemap should only contain URLs you wish for search engines to index. URLs in your sitemaps should be clean - i.e. sitemaps should only include URLs that are HTTP status 200 (OK), indexable, canonical and unique.
If search engines find 'dirt' in sitemaps, such as 500 pages, they may stop trusting the sitemaps for crawling and indexing signals.
Eric Enge once interviewed Duane Forrester while he was at Bing;
What does the Hint check?
This Hint will trigger for any internal URL which returns an HTTP status of 5XX, and is included in one of the submitted XML Sitemaps.
Examples that trigger this Hint:
Consider the URL: https://example.com/page-a, which is included in a submitted XML Sitemap.
The Hint would trigger for this URL if it had a 500 (Internal Server Error) header response:
Similarly, URLs with other 'server error' statuses will trigger the Hint (i.e. any other 5XX response).
How do you resolve this issue?
This Hint is marked 'Critical' as it represents a fundamentally breaking issue, which may have a serious adverse impact upon organic search traffic. It is strongly recommended that Critical issues are dealt with as a matter of high priority.
5XX errors indicate an issue with the server itself (rather than the website). Server errors affect access to your site, for search engines and users alike, and you really want to get to the bottom of why they occur - you may need to deal with a developer or server admin to understand what is causing the server errors.
Once the server errors are resolved, check that the XML Sitemap remains 'clean.'