It is probably because the User Manager (Public IP) is located on another place.
The HotSpot Server (Internal IP) is behind a modem.
<HotSpot> ---- <DSL modem> --- <internet> ---- <UserManager>
Since now, the User Manager aware of the modem's address and everything was working just fine (AAA, paypal...)
But the /tool user-manager session close-session command fails probably because the UM cannot access the HotSpot Server
at the known IP address (I'm not sure, I guess it).
Is there anything I can do to get over it? Port forwarding?