I never see this function the_post_thumbnail_caption();
, so change it by get_post( get_post_thumbnail_id() )->post_excerpt
<div class="slider">
<?php query_posts( 'post_type=page' ); ?>
<?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>
<?php the_post_thumbnail('slider'); ?>
<div class="caption-wrap"><?php echo get_post( get_post_thumbnail_id() )->post_excerpt ?></div>
<?php endwhile; else: ?>
<p><?php _e('Sorry, no posts matched your criteria.'); ?></p>
<?php endif; ?>
</div>