I've been using the Multiple Post Thumbnail plugin with my custom post types to use a secondary, smaller image on my archive pages, while I have a larger hero image in the header of the single post (these are two different images, not a smaller and larger version of the same image).

Previously, the code worked well, but suddenly it stopped returning the post thumb url.

The code in my functions.php file:

if (class_exists('MultiPostThumbnails')) {
    new MultiPostThumbnails(
        array(
            'label'     => 'Secondary Image',
            'id'        => 'secondary-image',
            'post_type' => 'episodes'
        )
    );
}
if (class_exists('MultiPostThumbnails')) {
    new MultiPostThumbnails(
        array(
            'label'     => 'Secondary Image',
            'id'        => 'secondary-image',
            'post_type' => 'resources'
        )
    );
}

The code in the loops on my custom post types' archive pages:

<?  $custhumburl = MultiPostThumbnails::get_post_thumbnail_url('episodes', 'secondary-image', NULL, 'secondary-image'); ?>

<div id="post-<?php the_ID(); ?>" <?php post_class('grid-box inline-item thumb') ?> itemscope itemtype="">

                <a href="<?php the_permalink(); ?>" itemprop="significantLink">

                <span class="display-block wp-post-image" style="background-image:url(<?php echo $custhumburl ?>);"></span>

                <span class="bg-gray-solid border-radius-bottom display-block lato all-padding">

                    <h3 class="center-text gray-text lato letter-spacing-heading sub-heading-font text-padding uppercase" itemprop="title"><?php the_subtitle(); ?></h3>

                    <h2 class="black-text capitalize center-text font-light heading-font roboto" itemprop="title"><?php the_title(); ?></h2>

                </span><!-- End post meta -->
            </a><!-- End post anchor tag -->
        </div><!-- End grid-box - post thumbnail -->

When I open Dev Tools, the image url is blank:

<span class="display-block wp-post-image" style="background-image:url();"></span>