Only one VPN connection at a time

Hi,

I have set up a vpn to windows server 2003.

The VPN connects fine for each client but for some reason, as soon as one client is connected, no others can connect and recieve a 720 error.

I have read a few posts on the interenet that suggest this could be an issue with the firewall only allowing one VPN connection through at a time.

Please could someone give me some advice on how to confirm if this is the issue or not or if I have something configured incorrectly.

Kind Regards

Ash