In the web page docs.endian.com/3.0/utm/vpn/server.html, section "Allow multiple connections from one account", is written:
"Normally, one client is allowed to connect from one location at a time. Selecting this option permits multiple client logins, even from different locations. However,
when the same client is connect twice or more, the VPN firewall rules do not apply anymore."
It's very shame there is only one client at time that can be connected properly through VPN firewall...