Proxy 502 error

Hi everybody,

I'm currently having a problem updating modules and installing them from an url. I have configured the proxy in the sites/default/settings.php:

$conf['proxy_server'] = 'proxy.company.com';
$conf['proxy_port'] = 3128;
$conf['proxy_username'] = '';
$conf['proxy_password'] = '';
$conf['proxy_user_agent'] = '';
$conf['proxy_exceptions'] = array('127.0.0.1', 'localhost');

When I try to update a module I'm getting the following error:

HTTP error 502 occurred when trying to fetch https://ftp.drupal.org/files/projects/XYZ.tar.gz.

What is the reason for this error? Hope you can help me.
Thank's a lot!