meta_query not working

I have a custom dropdown field in my media uploader which saves to the posts meta. Now I’m trying to get all images with a specific value with a meta_query. The variable $author is exactly the value that is saved in the postmeta in MySQL.

$rd_args = array(
    'meta_query' => array(
        array(
            'key' => 'post_slider_author',
            'value' => $author
        )
    )
);

$rd_query = new WP_Query( $rd_args );

$mh_posts = $rd_query->posts;

As you can see in the screenshot there is a value assigned to the meta post_slider_author and ‘1139’ is also the correct ID. Unfortunately the query stays empty.

Additional:
I tried to disable all plugins but it had no effect. Normal queries work just fine.