Compatibility issue with NextGEN Gallery plugin

Hello, thank you for your plugin and its constant updates!
I’m experiencing an issue related to the NextGEN Gallery plugin. I’ve already talked to the developer and – deactivating all the plugins and reactivating them one by one – we found out that the error is somehow related to WP Download Manager plugin. In fact it desappears when I deactivate WP Download Manager. So, when I go into a tag page, such as

http://www.rinoadiary.it/tag/i-am-setsuna/
or
http://www.rinoadiary.it/tag/tomb-raider/

I get this error:

Warning: Invalid argument supplied for foreach() in /web/htdocs/www.rinoadiary.it/home/wp-content/plugins/nextgen-gallery/products/photocrati_nextgen/modules/nextgen_gallery_display/package.module.nextgen_gallery_display.php

The 1852 line is

foreach ($display_type->entity_types as $type) {

1846-1866 lines:

public function is_compatible($source, $display_type)
    {
        $retval = FALSE;
        if ($source = $this->get($source->name)) {
            // Get the real entity type names for the display type
            $display_type_entity_types = array();
            foreach ($display_type->entity_types as $type) {
                $result = $this->get_entity_type($type);
                if ($result) {
                    $display_type_entity_types[] = $result;
                }
            }
            foreach ($source->returns as $entity_type) {
                if (in_array($entity_type, $display_type_entity_types, TRUE)) {
                    $retval = TRUE;
                    break;
                }
            }
        }
        return $retval;
    }

The error does NOT appear when viewing category pages.
Can you help?
Thank you!