HELP – Page content div closes after 5 galleries

This is what my filter output looks like

add_filter( 'post_gallery', 'my_post_gallery', 10, 2 );
function my_post_gallery( $output, $attr) {

$output = '
             <section class="gallery portfolio-square">
                <div class="portfolio-items-wrap">
                    <div class="flexslider carousel">
                        <ul class="slides">';
$images = explode(',', $attr['include']);
foreach ($images as $id){
    $attachment = get_post($id);
$output .= '
                            <li class="carousel-item fromblog-post portfolio-square">
                            <a href="'.wp_get_attachment_url( $id ).'" title="'.$attachment->post_excerpt.'">
                                '.wp_get_attachment_image( $id, "square").'
                            </a>
                            </li>  ';
}
$output .= '
                        </ul>
                    </div>
                    </div>
                </div>
            </section>  ';

//$output = print_r($images);
return $output;
}

in the posted code, you are opening two divs:

'section' 'div' 'div' 'ul'

but are closing three divs:

'/ul' '/div' '/div' '/div' '/section';

Thank you for the extra eyes. That fixed it.