plugin and theme conflict ?

I think there is a conflict between this plugin and my theme, a child of theme University. The plugin doesn’t display correctly. I’ve tryed to disable every plugin I use, nothing changes. I use WP 4.4.2.

I’ve added to Category.php <?php echo category_description( $category_id ); ?> before cat description and the picture is well displayed, no problem.

Now I want to display subcat picture on cat page, so I add to Category.php :

<ul>
 <?php foreach (get_categories() as $cat) : ?>
 <li>
 <?php z_taxonomy_image($cat->term_id); ?>
 <a href="<?php echo get_category_link($cat->term_id); ?>"><?php echo $cat->cat_name; ?></a>
 </li>
 <?php endforeach; ?>
</ul>

(I try too with “get_the_category” >> same result)

This is my site structure :
cat 1
subcat A
3 articles
subcat B
3 articles

cat2
subcat C
1 article
subcat D
1 article
subcat E
1 article

This is what happens :

On cat1 page, I see only subcatB picture
> the last article I add was in subcatB
> if I add a new article in subcatA, then I see only subcatA picture.

On cat2 page, I see only subcat E picture, because the last article I add was in subcatE
…

I think the plugin is in conflict with University theme, because I try with another theme and it works better.

Anyway, thanks for this plugin, though I am sad not to be able to use it.

https://ww.wp.xz.cn/plugins/categories-images/