I believe I found the main cause, but not the solution
if, for any reason, the router reboots, such as power lost - everyone on that router will be removed, and because
the UM manager had not received accounting stop commands, remove commands, the sessions stays active on the UM
and the users will not be able to re-log in...
ok, this is the cause I know 100%
is there a way for routers to send commands to UM at boot up to remove some user sessions so users can be able to
sorry, I though it was about Idle remove time, but I guess I was wrong...