Posting to an older thread but could be of use to someone. I had the same issue; it turned out to be caused by the theme which had this code in functions.php, which is intended to run on the frontend but is breaking search/filter on the backend. You see this limits results to only “posts”; since images are not posts.. no results whatsoever!
function gt_search_filter($query) {
if ($query->is_search) {
$query->set('post_type', 'post');
}
return $query;
}
I wrapped the offending line in another if statement to resolve
if (!is_admin()){ ... }
