You cannot add the filter to the_excerpt() it needs to be removed
// add_filter('the_excerpt', 'jqlb_autoexpand_rel_wlightbox', 99);
1. The excerpt gets filters of html images to begin with, there's no reason for it.
2. When themes utilize get_the_excerpt() for things like meta descriptions (or SEO plugins) your
return "\n<!-- wp-jquery-lightbox, a WordPress plugin by ulfben --> \n" . $content;
destroys the output.