Does it help to comment the login_after hook?
function init()
{
$this->add_texts('localization/');
if(file_exists("./plugins/captcha/config/config.inc.php"))
$this->load_config('config/config.inc.php');
else
$this->load_config('config/config.inc.php.dist');
$this->add_hook('template_object_captcha', array($this, 'show_captcha'));
//$this->add_hook('login_after', array($this, 'check_captcha'));
$this->add_hook('render_page', array($this, 'check_captcha'));
}