Hi Youri,
The session count will under normal circumstances count one up when a user logs in and one down when the user logs out or 20 minutes after they close their browser.
There is usually no problem when the user logs out using the log out using the orange X in the bottom right corner.
When users just close the browser, the Dynamic AI Server is not informed that the user is quitting, and has to rely on IIS own session_end event to execute the log off code that decrements the session counter.
On some computers (usually Windows 2003 Server) the IIS event does not have permissions to execute the Dynamic AI code. It usually helps to install all hot fixes for windows.
If this does not help, you can run IISreset to reset the counters (and memory etc) as Carsten said, and then send me details on your installation - OS version, IIS account info etc on
boa@cintac.com
Best regards
Bo