Pagination not working with custom category template

To go further in my knowledge of wordpress, I’ve been developing a theme from scratch, and using plugins ACF and CPT UI, in order to create a simple and intuitive backoffice interface.

My problem is on the front-end although. I’ve been creating pagination for the homepage (using a custom query) and it works perfectly. This is not the case when it comes to category pages (in a template named category.php).

I’ve been Googling for hours in order to find a solution, but everything seems related to permalinks, when I don’t have any problems with permalinks (I’m using the default structure /%postname%/ ).

Here’s the code for my category page, including the custom loop and pagination :

<?php
$paged = ( get_query_var('page') ) ? get_query_var('page') : 1;
$my_videos = new WP_Query(array(
   'post_type' => 'video', 
   'category_name' => single_cat_title('', false), 
   'posts_per_page' => 6
)); 
 if ( $my_videos->have_posts() ) :  
  while ($my_videos->have_posts()) : $my_videos->the_post();
   show_video_thumbnails();
  endwhile; 
else:  ?>
<p><?php _e( 'Sorry, no posts matched your criteria.' ); ?></p>
<?php endif; ?>

<!-- pagination here -->
<?php   next_posts_link( 'Older Entries', $my_videos->max_num_pages );
        previous_posts_link( 'Newer Entries' );
        wp_reset_postdata(); 
?>

Everything works great on the first page of the category: pagination is available and shows “Older entries”.

But when clicking the link, I’m being redirected to /category/mycategoryname/page/2 (mycategoryname being my category name), but it shows a 404 error;

Any help would be greatly appreciated. I feel like I’m missing something here.

As this is your main loop of your template, you should not be making a new loop, but modify the existing loop with pre_get_posts. This way you can be sure that all extra query parameters will be taken into account.

An example of how you would do this:

add_action( 'pre_get_posts', 'wpse5477_pre_get_posts' );
function wpse5477_pre_get_posts( $query )
{
    if ( ! $query->is_main_query() || $query->is_admin() )
        return false; 

    if ( $query->is_category() ) {
        $query->set( 'post_type', 'video' );
        $query->set( 'posts_per_page', 6 );
    }
    return $query;
}

This code will go in your functions.php.

First we check if this is the main loop and not your admin area. pre_get_posts can affect admin.

Then, if this is a category, we modify the $query.

And then return the $query.

For more information check here: https://codex.wordpress.org/Plugin_API/Action_Reference/pre_get_posts