Shortcode breaks Plugin

Hi there,

the Plugin works great an all my pages, except for the pages where I added a simple function via shortcode. The widgets just don’t show up. If I change the shortcode (“_get_special_posts”) the Widgets do appear.

The shortcode is this:

[get_special_posts cat=1 anz=3]

and in functions.php I have this:

function get_special_posts_function($atts) {
	$a = shortcode_atts( array(
        'cat' => '1',
        'anz' => 5
    ), $atts );
	query_posts('cat='.$a['cat'].'&showposts='.$a['anz']);

	echo $header;
                if (have_posts()) : while (have_posts()) :
                 
                 the_post();
                 $t = get_the_title();

                 echo "<h3>".$t."<h3>";
                
            endwhile;
        endif;
        }

add_shortcode( 'get_special_posts', 'get_special_posts_function' );  

What is wrong here? How can I fix this?

Thanks!