PHP Warning: session_regenerate_id(): Session object destruction failed

Ahoj,

jednou, dvakrat mesicne mi Tracy zapise do logu nasledujici chybu:

[2016-03-23 13-53-39] PHP Warning: session_regenerate_id(): Session object destruction failed.  ID: files (path: ) in C:\inetpub\wwwroot\web\vendor\nette\http\src\Http\Session.php:221  @  http://192.168.108.150/sign-in/

Jedna se o aplikaci, ktera bezi na firemnim intra netu (Windows Server 2008 R2, IIS 7, PHP 7.0.2). Chybu se mi nepodarilo nikdy duplikovat. Napada nekoho jak tento problem vyresit?

Takhle vypada prihlasovaci presenter, nic zvlastniho na nem neni:

<?php

namespace App\FrontModule\Presenters;

use App;
use App\Components\Forms\TemplateControl;
use Nette;
use Nette\Application;
use Nette\Security;


final class SignInPresenter extends BasePresenter
{
	/** @var App\FrontModule\Components\Forms\SignInFormFactory @inject */
	public $signInFormFactory;

	protected function startup()
	{
		parent::startup();
		if ($this->user->isLoggedIn()) {
			$this->redirect("Home:default");
		}
	}

	/**
	 * @return App\Components\Forms\TemplateControl
	 */
	protected function createComponentSignInForm(): App\Components\Forms\TemplateControl {
		$form = $this->signInFormFactory->create();

		$form->onSuccess[] = function(Application\UI\Form $form, array $values) {
			try {
				$this->user->login($values["email"], $values["password"]);

				if ($values["remember"]) {
					$this->user->setExpiration("+14 days", FALSE);
				} else {
					$this->user->setExpiration("+30 minutes", TRUE);
				}

				$this->redirect("Home:default");
			} catch (Security\AuthenticationException $e) {
				$form["email"]->addError("front.signInFormFactory.danger_1");
			}
		};

		return new TemplateControl($form, TemplateControl::TEMPLATE_BOOTSTRAP_V4);
	}
}