Custom Post Type, 404 error

Question

I created a very small plugin using a Custom Post Type.

This is NOT a commercial or publicly available plugin, only my website is using it.

What happens:

1. when I click the name of a user who posted a post, I should eventually get a page containing the list of the titles of the posts he posted: I get a 404 page;

2. when I click any monthly archive of posts I get a 404 page, EXCEPT when a document of my custom post type is present in the list;

When I disable my plugin, 404 error goes away.

I red anything about flushing permalinks, but nothing changed: once the plugin is activated, I get the 404 error.

I can eventually post all the code, but I noticed many posts where the relevant part is the following:

function add_DeltaDocuments_custom_post_type()
        {
            unregister_post_type('docs');

            $labels = $this->set_ui_labels('DeltaDocument', 'DeltaDocuments');

            register_post_type('docs', array(
                    'labels' => $labels,
                    'public' => true,
                    'has_archive' => true,
                    'menu_icon' => 'dashicons-text-page',
                    'exclude_from_search' => false,
                    'taxonomies' => ['category'],
                    'rewrite' => array('slug' => 'docs'),
                    'supports' => array('title', 'editor', 'author', 'thumbnail', 'excerpt', 'custom-fields', 'revisions'),
                      )
                );
        }

Any help is appreciated.

Answer 1

I solved by eliminating the following portion of code: I don't remember why I had to code it, it apparently solved an issue, even if I don't remember what issue.

The portion is the following:

add_action('pre_get_posts', function ($query) {
                if (!is_admin() && $query->is_main_query() && empty($query->query_vars['suppress_filters'])) {

        if (is_archive()) {

            if (is_category()) {
                $query->set('post_type', ['post', 'docs']);
            } else {
                if (!is_tag()) {
                    $query->set('post_type', 'docs');
                } else {
                    $query->set('post_type', 'post');
                }
            }
        }
    }
});

Maybe a newer version of WordPress had compatibility issues with this code.