The plugin is hard-coded to expect that the plugins path will contain wp-contents. WordPress finds it completely acceptable to move all kinds of directory locations around, including renaming the wp-contents directory. Aspects of this plugin break if you do that. Thankfully, it is easy to fix.
On line 36 of media_tags.php, change:
$this->plugindir_url = get_bloginfo('wpurl') . "/wp-content/plugins/". $plugindir_node;
$this->plugindir_url = WP_CONTENT_URL."/plugins/". $plugindir_node;
And it works fine.