Thanks to Ken, the problem was eventually solved by making the following 2 changes:

• in file \phpuploader\include_phpuploader.php:
  • change from $cd=dirname($this->GetWebPath(__FILE__)); 
  • to $cd=dirname($_SERVER['SCRIPT_NAME']).'phpuploader';
• in file creating the new instance of PhpUpLoader:
  • change from php require_once "phpuploader/include_phpuploader.php"
  • to require_once ($_SERVER["DOCUMENT_ROOT"] . "/archives/_admin/phpfileuploader/phpuploader/include_phpuploader.php");