What triggers this issue?
This issue reports all internal page URLs of a website returning one of the 4xx (Client Error) HTTP response codes that receive some organic search traffic (as per data from Ahrefs’ Site Explorer).
Why is it important?
4xx HTTP response codes indicate that there is an error in the request from the client. The most common 4xx errors are 404 (Not found) and 403 (Forbidden). Given that the page receives organic traffic, it might have changed its status to 4xx not so long ago. These pages will be removed from the search index only after search engines re-crawl them (which may take a while).
People landing on 4xx pages from search results will definitely not be satisfied and most likely will leave your website immediately. This provides poor user experience and might send negative user behaviour signals to Google.
You should note that organic search traffic in Ahrefs is not real-time data, and search engines might have already deindexed the 4xx URLs.
How to fix it?
URLs that return one of the 4xx response codes will be removed from Google's index upon a re-crawl.
You can use Fetch as Google tool in Google Search Console to ask Google to re-crawl the URL. This will speed up the de-indexing process.
If the page was deleted, moved, or blocked unintentionally, you should restore it.
You should also note that a DoS protection system or the firewall configuration can block access to the website from certain locations, making them return the 403 code to specific users. Also, these systems may be blocking our crawler specifically on a server level, while for visitors this page can be live.