Fix pagination after WordPress 5.5 update

Solution:

Temporary fix without touching the core: disable the 404 handling for paged urls via filter pre_handle_404 in your theme’s functions.php:

function pre_handle_404($preempt, $wp_query)
{
    if (isset($wp_query->query['page']) && $wp_query->query['page']) {
        return true;
    }

    return $preempt;
}
add_filter( 'pre_handle_404', 'pre_handle_404', 10, 2 );