Media handle sideload not working

I am using this media uploader wp snippet which downloads an image from my one site to other. It was working fine until yesterday but I checked this morning, it wasn’t working.

            require_once(ABSPATH . 'wp-admin/includes/file.php');
            require_once(ABSPATH . 'wp-admin/includes/media.php');
            require_once(ABSPATH . 'wp-admin/includes/image.php');

            $media_img_url="http://abc123.com/wp-content/2013/08/qwerty.jpg";
            $tmp = download_url( $media_img_url );
            $post_id = 32454;
            $desc="The image is here";

       echo $tmp;  //Output: /tmp/qwerty.tmp

// Set variables for storage
// fix file filename for query strings
preg_match('/[^\?]+\. (jpg|JPG|jpe|JPE|jpeg|JPEG|gif|GIF|png|PNG)/', $media_img_url, $matches);              
$file_array['name'] = basename($matches[0]);
$file_array['tmp_name'] = $tmp;

print_r($matches); //Output: Array()
echo $file_array['name'].                  //Output: Nothing
     '<br>'.$file_array['tmp_name'];       //Output: /tmp/qwerty.tmp


            // If error storing temporarily, unlink
            if ( is_wp_error( $tmp ) ) {
                @unlink($file_array['tmp_name']);
                $file_array['tmp_name'] = '';
            }

            echo $file_array['name'];        //Output: Nothing

            // do the validation and storage stuff
            $id = media_handle_sideload( $file_array, $post_id, $desc );

           print_r($id); //Output: WP_Error Object ( [errors] => Array ( [upload_error] => Array ( [0] => File is empty. Please upload something more substantial. This error could also be caused by uploads being disabled in your php.ini. ) ) [error_data] => Array ( ) ) 

            // If error storing permanently, unlink
            if ( is_wp_error($id) ) {
                @unlink($file_array['tmp_name']);
                return $id;
            }

          print_r($id);                  //Output: Nothing

        $src = wp_get_attachment_url( $id );

          print_r($src);                  //Output: Nothing

For debugging purposes, I echoed values at some points to know where it is not working. I have checked php.ini file many times,nothing is changed.
Here are errors:

[Sun Sep 01 07:06:46 2013] [error] [client 213.236.00.15] (70014)End of file found: Error reading request entity data
[Sun Sep 01 07:11:09 2013] [error] [client 213.236.00.15] ModSecurity: Error reading request body: End of file found [hostname "abc123.com"] [uri "/wp-admin/admin-ajax.php"] [unique_id "UiNK-dBhnx0AAA3rD4cAAAAC"]

Frontend uploading Visual:
Error when uploading images from frontend:

[Sun Sep 01 22:54:18 2013] [error] [client 213.236.00.15] ModSecurity: Input filter: Failed writing 8192 bytes to temporary file (rc 18446744073709551615). [hostname "abc123.com"] [uri "/wp-admin/async-upload.php"] [unique_id "UiQoCdBhnx0AACtXBFwAAAAA"]
[Sun Sep 01 22:54:55 2013] [error] [client 213.236.00.15] ModSecurity: Multipart parsing error: Multipart: writing to "/tmp/20130901-225455-UiQoL9Bhnx0AACtYB4gAAAAD-file-pwzC5a" failed [hostname "abc123.com"] [uri "/wp-admin/async-upload.php"] [unique_id "UiQoL9Bhnx0AACtYB4gAAAAD"]

These two lines are present redundantly with different timings in error log.

Please anyone suggest how to resolve this?
Thanks.

For those having similar problem, I cleared web-server’s /tmp partition
being filled with temp files. The root cause was actually that my hosting provider sets the /tmp partition to be 128MB when on their VPS, which is far too small for the size of uploads that I want to be able to do.