错误502使用CodeIgniter时网关错误

下面的代码是从我的控制器上运行的，它在一台服务器上导致502错误的坏网关。我无法在服务器上复制。502坏网关的一些原因是什么？

function index()
{
    $this->_reload();
}
function _reload($data=array())
{
    $person_info = $this->Employee->get_logged_in_employee_info();
    $data['cart']=$this->sale_lib->get_cart();
    $data['modes']=array('sale'=>$this->lang->line('sales_sale'),'return'=>$this->lang->line('sales_return'));
    $data['mode']=$this->sale_lib->get_mode();
    $data['items_in_cart'] = $this->sale_lib->get_items_in_cart();
    $data['subtotal']=$this->sale_lib->get_subtotal();
    $data['taxes']=$this->sale_lib->get_taxes();
    $data['total']=$this->sale_lib->get_total();
    $data['items_module_allowed'] = $this->Employee->has_permission('items', $person_info->person_id);
    $data['comment'] = $this->sale_lib->get_comment();
    $data['email_receipt'] = $this->sale_lib->get_email_receipt();
    $data['payments_total']=$this->sale_lib->get_payments_total();
    $data['amount_due']=$this->sale_lib->get_amount_due();
    $data['payments']=$this->sale_lib->get_payments();
    $data['payment_options']=array(
        $this->lang->line('sales_cash') => $this->lang->line('sales_cash'),
        $this->lang->line('sales_check') => $this->lang->line('sales_check'),
        $this->lang->line('sales_giftcard') => $this->lang->line('sales_giftcard'),
        $this->lang->line('sales_debit') => $this->lang->line('sales_debit'),
        $this->lang->line('sales_credit') => $this->lang->line('sales_credit')
    );
    $customer_id=$this->sale_lib->get_customer();
    if($customer_id!=-1)
    {
        $info=$this->Customer->get_info($customer_id);
        $data['customer']=$info->first_name.' '.$info->last_name;
        $data['customer_email']=$info->email;
    }
    $data['payments_cover_total'] = $this->_payments_cover_total();
    $this->load->view("sales/register",$data);
}