404 on subsub page

What happens if you requested website.com/subsubpage/? Expected result is the page in question is shown and the browser address field is rewritten to website.com/mainpage/subpage/subsubpage/

Your site’s permalink setting is just /%postname%/, right?

This 404 you get is the WordPress themed nothing found sort and not a generic, plain server 404 page, correct? If so, the related query for the page has somehow failed. Install and activate the Query Monitor plugin. After requesting the subsubpage, filter the queries in the monitor for caller get_page_by_path().

The query should find 3 rows. The data returned is used to determine the ID of subsubpage, which is used in the main query to get the page. If 3 rows were not found, there’s something off with the query made, like additional, unneeded conditions that cannot be met. Any such unneeded conditions could be a clue to the root cause. The query should be something like

SELECT ID, post_name, post_parent, post_type
FROM wp_posts
WHERE post_name IN ('mainpage','subpage','subsubpage')
AND post_type IN ('page','attachment')

Yes, it’s all /%postname%/. They used to work. But somehow they stopped working.

The 404 page is a the custom one I made.

Installed the plugin, but it couldn’t find the part.

Tried out some stuf, and it looks like there is a problem with the particular mainpage. I have some other pages with the same path ‘length’, and that path doesn’t have the problems.

Just deleted the corrupt mainpage. Seems like everything is working allright again.

Back again. Now all of my subsubpages get a 404 error when I try to visit them…

Tried the Query Monitor plugin and tried to find the named part, but that doesn’t show.

Build another website, but the same problem turned up. I build the website with the subsubpages, worked fine untill last week. When I build the same setup with new pages, it’ll work. But when I rename them, and the slug, to the pages I want, the subsubpage shows a 404.