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Problem:
If you're experiencing a 404 error when switching between language versions using category-based URLs on your multilingual WordPress site with WPML, and you have already linked all categories correctly, flushed permalinks, and cleared cache, the issue might still persist.
Solution:
We recommend the following steps:
1. Contact your hosting provider to ensure the mod_rewrite Apache extension is active.
2. Create a full site backup.
3. Update any outdated plugins or themes.
4. Flush any existing cache memory.
5. Deactivate all plugins, including WPML ones, and activate a default WordPress theme.
6. Activate WordPress debug mode as described in this guide: Debug WordPress
7. Reactivate the WPML plugins and re-save the permalinks.
8. If the issue is resolved, reactivate your theme and plugins one by one to identify the culprit.
9. Increase the WP memory limit to at least 128Mb as detailed here: WPML Minimum Requirements
Add the following code to your wp-config.php:

/* Memory Limit */<br />define('WP_MEMORY_LIMIT', '256M');

Place it above the line /* That's all, stop editing! Happy blogging. */
You can verify the WordPress memory limit in WPML -> Support.

If these steps do not resolve your issue, or if the solution seems outdated or not applicable to your case, we highly recommend checking related known issues at https://wpml.org/known-issues/, verifying the version of the permanent fix, and confirming that you have installed the latest versions of themes and plugins. If needed, please do not hesitate to open a new support ticket at WPML Support Forum for further assistance.