I downloaded it, was at first disappointed about that simple error. Here is what fixed it for me :
The second last line of mod_categories.php shows :
HTML_content::showContentList( $section, $null, $access, $id, $null, $gid, $params, $null, $categories, $null, $null);
After checking in content.html.php I found out that there is a parameter missing. Just add " , $null" (without ") inside the bracket as last parameter to show this :
HTML_content::showContentList( $section, $null, $access, $id, $null, $gid, $params, $null, $categories, $null, $null, $null );
and it works.