[Plugin: Thumbnail For Excerpts] TNs not showing on pages including posts/showing wrong thumbnails

Hello there,
I am experiencing some problems with the thumbnail-for-excerpt plugin. It really works well on category pages, but refuses to work on pages including post, e.g. showing the last five comments from three categories. The symptoms are: no thumbnails are shown normally. If the content of the page itself contains a picture, TNFE uses that one as the thumbnail for all posts included on that page instead of the single ones from the posts themselves. The actual page content can there be inserted as either the_content() or the_excerpt(), it doesn’t matter.

Here is, more or less, the relevant part for the page in code:

<?php
/*
Template Name: Portfolio
*/

get_header(); ?>

<?php get_sidebar(); ?>

<div class="left portfolio">
            <h2 class="contentheading"><?php the_title(); ?></h2>
            <ul>
 <?php wp_list_categories('title_li=&exclude=9,10&child_of=1'); ?>
</ul>
<p class="fatspacer"></p>
<?php the_content(); ?>
<p class="fatspacer"></p>

<?php
// page id 21 will get category ID 12 posts, page 16 will get category 32 posts, page 28 will get category 17 posts
if (is_page('18') ) {
$cat = array(6,7,8);
} elseif ( is_page('126') ) {
$cat = array(9);
} elseif ( is_page('22') ) {
$cat = array(10);
} else {
$cat = '';
}

$showposts = 5; // -1 shows all posts
$do_not_show_stickies = 1; // 0 to show stickies
$args=array(
   'category__in' => $cat,
   'showposts' => $showposts,
   'caller_get_posts' => $do_not_show_stickies
   );
$my_query = new WP_Query($args); 

?>
	<?php if( $my_query->have_posts() ) : ?>

		<?php while ($my_query->have_posts()) : $my_query->the_post(); ?>
			<?php
			//necessary to show the tags
			global $wp_query;
			$wp_query->in_the_loop = true;
			?>

… followed by the elements for the excerpts etc.
Does anybody have a basic idea what could hinder TNFE from seeing the appropriate thumbnails and inserting them? I’d be grateful if someone could provide help….

Cheers!

https://www.ads-software.com/extend/plugins/thumbnail-for-excerpts/