Many Wordpress users encounter the problem where setting custom permalinks for custom post types suddenly results in a 404 error when trying to access these posts. This issue can stem from several factors, including incorrect permalink settings or conflicts with other plugins.

Root Causes

When setting custom permalinks for custom post types, it's crucial to ensure that the URL rewriting rules are properly defined and that they undergo proper flushing. If WordPress fails to register the new rewriting rules, requests to these posts might be directed to the wrong URL, resulting in a 404 error.

Resolving the Issue

1. Flushing Permalinks One of the most common solutions is manually flushing the rewrite rules. This can be done simply by navigating to Settings > Permalinks in your WordPress dashboard and clicking the "Save Changes" button without making any alterations. This step should automatically clear and re-register all rewriting rules.

2. Checking for Plugin Conflicts If the issue persists, it's recommended to check for conflicts with other plugins. You can do this by deactivating all plugins except for the one responsible for the custom post types and then testing whether the 404 error issue persists.

3. Correct Code Setup In your theme or plugin that adds custom post types, ensure that you have the 'rewrite' argument properly set when registering the custom post type. Here's an example code for registering a custom post type with a custom permalink:

register_post_type('custom_type',
    array(
        'labels' => array(
            'name' => __('Custom Types', 'textdomain'),
            'singular_name' => __('Custom Type', 'textdomain')
        ),
        'public' => true,
        'has_archive' => true,
        'rewrite' => array('slug' => 'custom-types'),
    )
);

After adding or changing the code, don't forget to flush the permalinks again.

404 error issues after setting custom permalinks can be frustrating, but in most cases, they are solvable through simple steps like flushing permalinks, checking for plugin conflicts, and ensuring correct code setup. These steps should ensure that your custom post types are properly accessible without unnecessary 404 errors.