What triggers this issue?
This issue reports canonicalized URLs where canonical links point to URLs that return one of the 5xx (Server Error) HTTP status code.
Example:
For https://help.ahrefs.com/en/
<head>
...
<link rel="canonical" href="https://ahrefs.com/help/en/"/>
...
</head>
And https://ahrefs.com/help/en/ declared as canonical, returns any of the 5xx HTTP status codes (Server error).
Why is it important?
Canonical links are used to solve duplicate content issues. If you have several pages with the same or similar content, you need to pick the most authoritative (canonical) version.
If the canonical page is not available for the search engines, they do their best to pick the canonical page on their own. And you might not be satisfied with their choice.
How to fix it?
Review the list of pages with the canonical link pointing to a 5xx URL. If erroneous URL was specified as canonical, replace it with the link to the valid 200 page version you want to be indexed in search results.
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 the 5xx errors are:
Faulty plugins in your CMS (such as Wordpress).
Permissions issues with your files and folders.
Some part of your website can be blocking our crawler (because of a firewall, DoS protection system, traffic blocks from certain countries, or a CMS configuration). Thus your website URLs may return 5xx codes to our crawler specifically.
If you found that the crawl happened during server maintenance, run a new crawl to see if the 5xx errors persist.
Another possible reason for this issue in the Site Audit tool is the high crawling speed set in your project's settings. Some servers are not able to process a large number of requests at a given time. Try setting lower crawling speed in the crawl settings and run a project re-crawl.