Since implementation of EZP-68, it's impossible to login any more in the admin interface (using legacy).

Seems to be related to HttpFoundation\Session that uses a session handler extending native \SessionHandler.
See https://github.com/symfony/symfony/blob/master/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/NativeSessionHandler.php .
Note that commenting out the PHP 5.4 part in this file makes everything work as expected.

Module user/login probably makes something bad with sessions.