Hi Luca,
The Help says:
A ServerSession Web Property is attached to the session (expires together with the session). Its value stays on the server and is not accessible on the client.
Do you have a lot of abandoned, unexpired sessions? The above suggests that a regularly run process which uses the WebAppSessionManager to Expire timed out sessions should fix the issue. Maybe a WebService called via a Windows App (or even Powershell) run by Task Scheduler would do the job?