What triggers this issue?
This issue reports all URLs in the website’s sitemap files that return one of the 5xx (Server error) HTTP status codes.
Why is it important?
Sitemap file of a website must list all the pages you want search engines to crawl and index. Besides, Google uses sitemaps data as one of the factors when selecting canonical URLs.
5xx URLs in sitemaps cannot be accessed by crawlers. This can result in search engines, such as Google, ignoring your sitemaps. In this case, you might end up with some indexability issues on your website.
How to fix it?
Your sitemap files must only include the live URLs that return the 200 (OK) response code.
5xx (Server error) HTTP status codes indicate some server issues, and you should address your developer or hosting provider. Your server may be misconfigured, overloaded, or generally slow.
There are different types of server errors, and they must be addressed in different ways. The most common 5xx errors are 500 (Internal Server Error) and 503 (Service Unavailable).
500 error needs further investigation. It is usually caused by errors in application code or config files. Corrupted .htaccess file can often be the reason. PHP memory limit is another possible problem causing this error.
503 errors are temporary issues. They are usually caused by server overload or maintenance. These errors can indicate that you need a stronger web server, able to handle a bigger number of requests.
Other possible reasons for 5xx errors are:
Faulty plugins in your CMS (such as Wordpress)
Permissions issues with your files and folders
If you found that the crawl happened during server maintenance, run a new crawl to see if the 5xx errors persist.