I help manage https://www.delscookingtwist.com/, a bilingual food blog which runs WordPress (need I say that?) along with several other plugins including Yoast SEO and WPML.

Recently, I noticed 404 error reports in Google Analytics. I traced the beginning of these 404 error reports to plugin updates I ran on 12/8.

This blog post in English works fine but its French translation yields a 404. Yet the post is published and the slug is correct. I tried accessing the post with the page_id=xxx and it also 404ed. If I search for the post using WordPress’s search feature on the blog, I can see it, along with feature image, in the search results.

To narrow down the culprit, I made a copy of the site backup (before upgrade) to my staging environment and started updating each plugin one by one, testing whether a 404 occurred as a result. Eventually I narrowed down the issue to Yoast SEO Multilingual. This is the plugin that causes the 404.

You’d think it’d be the end of the story – roll back to the older version of the plugin and notify WPML, right? Well this is where things get weird: this plugin is actually inactive in both the prod site and the staging site (because I’d had issues with it prior)

So I’m pretty concerned at this point. Can a plugin install / update modify other plugins? Is there any kind of isolation to prevent that? Obviously, plugin installs / updates can modify the DB schema. So does this mean this plugin updated parts of the DB used by its sister WPML plugins?

I know this isn’t an ideal question. At this point, I’m looking for pointers. Thanks all!