WP_Query ignores post_type in category view

I have following query to select posts by my custom post-type. This works fine while being on the frontpage (the snippet is included in the sidebar). As soon it gets executed on a category page the post_type attribute is ignored and posts of the type “post” are returned. I guess this has to do with multiple loops but I have also tried to call wp_reset_postdata(); and wp_reset_query(); before and after.

$customPosts = new WP_Query( array( 
    'post_type' => 'mycustomtype', 
    'posts_per_page' => 12, 
    'orderby' => 'date', 
    'order' => 'DESC', 
    'post_status' => 'publish'
));

while ( $customPosts->have_posts() ) : $customPosts->the_post(); 
    $thumbnailUrl = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), "thumbnail");
    if ( !empty($thumbnailUrl)) {
      /* echo result */ 
    }   
endwhile;   

Have you tried using get_posts() instead?

 //#get access to post settings
    global $post;
    //#set parameters for extra loop
    $args = array(
    'post_type' => 'mycustomtype', 
    'posts_per_page' => 12, 
    'orderby' => 'date', 
    'order' => 'DESC', 
    'post_status' => 'publish'
    );
    //#get posts 
    $customPosts = get_posts($args);
    //#loop through them
    foreach($customPosts as $post)
    {
        //#set all the loop functions to use data from this post
        setup_postdata($post);
        //#do what you want with the post
    }