Symfony 2: session_start causes ERR_EMPTY_RESPONSE - session

Question

We just moved our Symfony2.3 project to another server and now we encounter the following problem:
The routes /login and _wdt/9d5405 result in a ERR_EMPTY_RESPONSE error in Google Chrome (and all other browsers showing their specific error messages). The error occurs in both, prod and dev environment.
I tracked down the issue from my loginAction
/**
* #Route("/login", name="login")
* #Route("/login/{redirect}", name="login_redirect")
* #Template()
*/
public function loginAction($redirect = '')
{
if ($this->getUser())
{
return $this->redirect($this->generateUrl('crea_frontend_index'));
}
$request = $this->getRequest();
$session = $request->getSession();
// get the login error if there is one
if ($request->attributes->has(SecurityContext::AUTHENTICATION_ERROR))
{
$error = $request->attributes->get(
SecurityContext::AUTHENTICATION_ERROR
);
}
else
{
$error = $session->get(SecurityContext::AUTHENTICATION_ERROR);
$session->remove(SecurityContext::AUTHENTICATION_ERROR);
}
$redirect = urldecode($redirect);
return array(
'last_username' => $session->get(SecurityContext::LAST_USERNAME), // last username entered by the user
'redirect' => $redirect,
'error' => $error,
);
}
This line produces the error:
$error = $session->get(SecurityContext::AUTHENTICATION_ERROR);
And I tracked it down to
Symfony/Component/HttpFoundation/Session/Storage/NativeSessionStorage.php
and there in line 148 if (!session_start()) {.
So it turnes out that session_start at this point produces the error but when I add a session_start(); to app.php and comment everything else it works.
Now I am completely puzzled on how to fix this. Any ideas on how to solve this or just how to further debug would be very appriciated.
PHP 5.4.20
No errors in logfiles
UPDATE
The error seems to be somehow related to these bug discussions:
https://bugs.launchpad.net/ubuntu/+source/php5/+bug/424789
https://bugs.gentoo.org/show_bug.cgi?id=276583
UPDATE
Solved: The Problem was the PHP Version 5.4.20. We tested 5.3.17 and 5.5.9. Both function well. Except 5.4.20. But I still want to know what exaclty causes this error. This could then result in a Symfony fix to deal with this version.