The problem with pagination was fixed by removing the wp_query loop and using the normal loop.
<?php if (have_posts()) : while (have_posts()) : the_post(); ?>
<?php endwhile; endif; ?>
The problem with the “load more” plugin was fixed by using the right classname to load the items. And change this in the php of the plugin.
$paged = ( get_query_var('paged') > 1 ) ? get_query_var('paged') : 1;
$args = array('post_type' => 'inbeeld', 'paged' => get_query_var('paged') );
$query = new WP_Query($args);
$max = $query->max_num_pages;