public $config = array(
    "upload_path" => "./uploads/",
    "allowed_types" => "gif|jpg|png",
    "max_size" => 100,
    "max_width" => 1024,
    "max_height" => 768,
    "encrypt_name" => TRUE
  );

public function __construct()
{
    $this->load->library("upload", $this->config);
}

public function store()
{
    // este é só o trecho onde o erro acontece...
    if (!$this->do_upload()) {
      $this->session->set_flashdata("error", $this->upload->display_errors());
      redirect("events/create");
    }
}

public function do_upload()
  {
    if (!$this->upload->do_upload("cover")) {
      return FALSE;
    }
    return $this->upload->data("file_name");
  }