The href attribute you are passing under the_post_thumbnail() is incorrect. Note that this attribute is applied to the img tag which obviously doesn’t supports href attribute.
It would be better to wrap the_post_thumbnail() with an <a> tag. So it will look something like the following:
echo '<a href="' . get_permalink( $thumbnail->ID ) . '">' . get_the_post_thumbnail( $post->ID, 'thumbnail', array( 'class' => 'group1 img-responsive' ) ) . '</a>';