I use JDBCSessionIdManager/JDBCSessionManager from Jetty to store my sessions in a DB. Jetty tries to be smart and determines if a session is dirty and only persists it then. The logic is that a call to HttpSession#setAttribute may cause the dirty flag to be set if A) the value is new OR B) the value is changed, i.e. not #equals to the previous value. Wicket stores a PageStoreManager$SessionEntry as an attribute. This class doesn't have a specific equals implementation and so from the Jetty point of view the session never changes. The effect is that after Jetty restores the session from the DB it will be always in the initial state.
One possible fix is to always return false in SessionEntry#equals. But I'm not sure what Jetty is doing here is correct either.