Featured image on post edit page not loading over HTTPS

It turned out to be a problem with the uploads directory (not sure what caused it initially). I was able to resolve the issue with the following filter:

function fix_ssl( $url ) {
    if ( is_ssl() ) {
        if (stripos($url, 'http://') === 0) {
        $url="https" . substr($url, 4);
        }
    }
    return $url;
}

function uploadDir($uploads) {
    $uploads['url'] = fix_ssl($uploads['url']);
    $uploads['baseurl'] = fix_ssl($uploads['baseurl']);
    return $uploads;
}

add_filter('upload_dir', 'uploadDir');