I was going crazy trying to solve this error. Switching plugins on and off, checking plugin settings, checking permalinks were set properly…. No matter what I did, my sites showed hundreds of 404 errors generated by visitors and search engines looking for site content via tags and categories. It turns out it is a common bug with WordPress 3.1 +.
Today, I found the solution.
As silly as this sounds, this is the first day that I thought to google for a solution. This is because I only fully noticed these particular 404s in my logs after I installed SEO Ultimate and set it to remove URL bases for categories and tags; for example to turn https://journalxtra.com/category/example/ into https://journalxtra.com/example/. I tweaked my permalinks this way because it seemed like a good idea at the time.
My mistake was to undo the tweak a week later. I was hit with thousands of 404’s for missing categories. Anyway, I ignored it thinking it would go away. And it did, to a point. I’m now getting only a few hundred missing category 404s every couple of days. So, how do you fix category and tag 404s?
The solution came from this page at WP Code Snippets. That solution has been packaged into a plugin available from the WordPress repository, here. Get it, activate it and leave it running until WordPress releases a permanent fix.
After activating the plugin my category and tag URLs might not look so pretty but they do at least work and I’m not getting 404’s for them.
Bootnote
The plugin creates a bug for improperly formatted redirections. Any redirection that with a source URL that lacks a trailing slash will lead to a 404 page while the WCS Custom Permalinks Hotfix plugin is active. For example, example.com/go/help will lead to a 404 but example.com/go/help/ will redirect to its intended target. You might use the Search Regex plugin to locate improperly formatted URLs within posts and correct them (after locating them by their 404s or the Redirection plugin (if that’s what you used to create them).