RRAS only allows 1 VPN session

I have a Windows 2003 Small Business server where I have configured PPTP VPN access. I opened the ports on the firewall and it seems to work fine – at least for the first session. Subsequent sessions are denied access with error 671 (I think that’s right) – Host computer not responding. After 5-10 minutes after the first session logs off, another user will be able to establish a VPN connection. I have increased the number VPN ports from 5 PPTP to 15 – and up to 15 L2TP for good measure even though we’re not using L2TP right now. What could be limiting my PPTP sessions to one at a time??

Thanks,
Jon