wordpress recursive zip folder file download php / path issue - php

Question

I know there is quite some code out there about that topic. But I can't get the code I found to work...
I'm using a function that comes from a Wordpress plugin called Zip-Attachments.
So I modified it to zip files from a specific folder. BUT it doesn't work....
It seems as if I'm missing something when it comes to accurately assigning a paths within the RecursiveDirectoryIterator class.
I was able to zip a single file and downloading it without the recursiveIteration functionality and passing the absolute path as a string. So the main part of the function works.
What am I missing to make that function work?
Here is my non working code:
function za_create_zip_callback(){
$upload_dir = wp_upload_dir();
$rootPath = $upload_dir['basedir'];
$upload_dir_Knippsbox = 'Knippsbox';
// Prepare File
$file = tempnam($upload_dir['path'], "zip");
$zip = new ZipArchive();
$zip->open($file, ZipArchive::OVERWRITE);
// create recursive directory iterator
$files = new RecursiveIteratorIterator (new RecursiveDirectoryIterator("{$rootPath}/{$upload_dir_Knippsbox}/"), RecursiveIteratorIterator::LEAVES_ONLY);
// let's iterate
foreach ($files as $name => $fileX) {
$filePath = $fileX->getRealPath();
$zip->addFile($filePath);
}
//Close the file
$zip->close();
// Add a download to the Counter
global $wpdb;
$meta_name = "_za_counter";
// Retrieve the meta value from the DB
$za_download_count = get_post_meta($postId, $meta_name, true) != '' ? get_post_meta($postId, $meta_name, true) : '0';
$za_download_count = $za_download_count + 1;
// Update the meta value
update_post_meta($postId, $meta_name, $za_download_count);
// We have to return an actual URL, that URL will set the headers to force the download
echo zip_attachments_url."/download.php?za_pretty_filename=".sanitize_file_name($pretty_filename)."&za_real_filename=".$filename;
die();}
Desperately looking for your expert views...
Thanks,
Ben