Is there something special I need to do, because I have three applications on Symfony 2.0 and after the login I can use any of those apps, but one Symfony 2.1 app does not work (does not authenticate) and it asks me to login. If I do that, Symfony 2.0 apps are logged out.
Has the authentication system changed radically? How do I adapt?
Yes
Security SF 2.0http://symfony.com/doc/2.0/book/security.htmlSecurity SF 2.1http://symfony.com/doc/current/book/security.htmlSecurity system is not downwardly compatible.
Many thanks to Stony, Upgrade How To: https://github.com/symfony/symfony/blob/master/UPGRADE-2.1.md#security
By default, Symfony2 relies on a cookie (the Session) to persist the security context of the user. So maybe SF2.1 login overwrites your cookie and destroies previous created sessions.
Have a look at Stateless authentication http://symfony.com/doc/2.0/book/security.html#stateless-authentication