User logoff fails under WebSphere 7

Under WebSphere 7, after creating a new database and importing xwiki-enterprise-ui-all-4.5.1.xar, I'm not able to logoff.

What I notice are the cookies behaviour:

Using xwiki 3.5 (in production):

1. when I access my instance of xwiki 3.5, if I open the login page, my browser send JSESSIONID and language cookies.
2. after login, my browse adds and sends the following cookies: username, password, rememberme and validation (all expires at end of session)
3. if i press logoff, my browser recevives the same last 4 cookies but they expire in a earlier date (1-jan-1970)
4. after the point 3. my browser don't have anymore the cookies username, password, rememberme and validation, so the user is logged off

But when I do the same actions with xwiki 4.5.1, what change is the point 3

3. if i press logoff, my browser recevives the same last 4 cookies but they expire at end of session, so the user isn't logged off