The file

/modules/mod_articles_category/mod_articles_category.php

Pulls in the parameter "item_heading" on line 69. This is expected to be, and in the past was, an integer of the heading level (defaults to 4).

$item_heading               = $params->get('item_heading');

Currently, when you create an articles_category module, the parameter (found under advanced tab) has been changed to header tag (param name "header_tag"). This is instead a string representation of the header to use, e.g. "h4".

This makes the module produce erroneous HTML.

E.g.

<div class="moduletable">
    <h3>Test Module</h3>
    <ul class="category-module">
        <li class="">
            **<h>**
                <a class="mod-articles-category-title " href="/latest-news/open-days.html">
                    Open days and events
                </a>
            **</h>**    
        </li>
    </ul>
</div>