mayonis on "Random Error 500 When Using "upload_dir" Filter"

Hi,

I get random error 500 and error 404 when I use upload_dir as below:

add_filter('upload_dir', 'lean_upload_dir');
function lean_upload_dir( $upload ){
	$upload['url'] = '/files';
	$upload['baseurl'] = '/files';

	return $upload;
}

See the issue here:
http://passionindesign.com/2014/05/15/安静/

Mostly, everything is working fine when you first visit the link.
And then every time you refresh the page, the photos are randomly broken with error 500 or 404. (Please try refresh the page a few times and you will see different images are broken everytime you refresh)

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These information might be helpful:
My WordPress installed on a shared hosting, with virtual memory 512mb

I think my host provider added this line in my .htacces: "suPHP_ConfigPath /home/my_root_folder/public_html"

in "my_root_folder", there is a php.ini (memory_limit = 256M)