Symfony 2 receiving anonymous token after basic authentication

I have a Symfony 2 app using the basic in_memory authentication (as described in the security documentation). The login works fine in our development environment(s). But on the staging server, the basic authentication doesn't seem to provide a proper token -as seen in the hereby provided logfile-; thus we keep on getting the login popup again and again.

Our security configuration:

security:
    firewalls:
        secured_area:
            pattern:    ^/
            anonymous: ~
            http_basic:
                realm: "Secured Demo Area"

    access_control:
        - { path: ^/admin, roles: [ROLE_ADMIN]}

    providers:
        in_memory:
            users:
                admin: { password: admin, roles: 'ROLE_ADMIN' }

    encoders:
        Symfony\Component\Security\Core\User\User: plaintext

This is the log output from the (successful) development environment login:

[2011-07-21 13:49:48] security.DEBUG: Read SecurityContext from the session [] []
[2011-07-21 13:49:48] security.DEBUG: Reloading user from user provider. [] []
[2011-07-21 13:49:48] security.DEBUG: Username "root" was reloaded from user provider. [] []

And this is the log output from the staging environment login:

[2011-07-21 13:53:08] security.INFO: Populated SecurityContext with an anonymous Token [] []
[2011-07-21 13:53:08] security.DEBUG: Access denied (user is not fully authenticated); redirecting to authentication entry point [] []
[2011-07-21 13:53:08] security.DEBUG: Calling Authentication entry point [] []

Thanks in advance for the help.