500 Server Error with Nginx and callback_after_upload

I get "NetworkError: 500 Internal Server Error - http://www.somedomain.com/upload_file/image1"

using this function below, on my local machine it works (apache web server) but not on the live site with Nginx!!!

function create_thumbnails($uploader_response,$field_info, $files_to_upload)

{

$this->load->library('image_moo');

//Is only one file uploaded so it ok to use it with $uploader_response[0].

$file_uploaded = $field_info->upload_path.'/'.$uploader_response[0]->name;

$thumbnail1 = $field_info->upload_path.'/thumb/thumb-'.$uploader_response[0]->name;

$this->image_moo->load($file_uploaded)->resize_crop(105,75)->save($thumbnail1,true);

return true;

}