the update-check script uses METHOD CONNECT.
this was not allowed in squid.

in squid.conf i had to put:

[…]
acl PORT_80 port 80            #allow port 80
[…]
http_access allow CONNECT PORT_80

if i run updatecheck, i get the following error

root@i-doit:/var/www/i-doit# ./updatecheck

• Loading module: updatecheck

  • Checking… Please wait..
    Error while connecting / cURL error: 56 - Received HTTP code 403 from proxy after CONNECT
    Make sure this host is connected to the internet!
    Proxy settings can be configured in src/config.inc.php
    Proxy configuration:  (
      'active' => true,
      'host' => '192.168.XXXXX',
      'port' => '3128',
      'user' => '',
      'pass' => '',
    )

  extract of my config file:
  root@i-doit:/var/www/i-doit/src/config.inc.php

  […]
  /**
    * @desc Proxy
    * –------------------------------------------------------------------------
    *      Proxy configuration
    */
  $g_proxy = array(
          "active" => true,
          "host"  => "192.168.XXXX",
          "port"  => "3128",
          "user"  => "",
          "pass"  => ""
  );
  […]

  any suggestions?