multiple pptp connections

Do you have more than 1 WAN Miniport configured ?
What are you running as your VPN gateway.
It sounds like you either don't have enough virtual pptp ports, or they are not configured properly

PPTP-based remote access VPN

You can use Windows 2000 remote access to provide access to a corporate intranet for remote access clients who are making PPTP connections across the Internet. If you want your remote access server to support multiple PPTP connections, complete the following steps:

Configure the connection to the Internet.

Configure the connection to the intranet.

Configure the remote access server as a corporate intranet router.

Configure the remote access server for PPTP clients.

Configure the PPTP ports.

Configure multicast support.

Configure PPTP filters.

Configure remote access policies.

The following illustration shows the elements of a Windows 2000 remote access server that provides PPTP-based remote access to a corporate intranet:

See link:

http://www.microsoft.com/windows2000/en/server/help/default.asp?url=/windows2000/en/server/help/sag_RASS_scen_pptp_rc.htm

(Remove spaces from the pasted URL.)

How do YOU access the internet !! ??
I had the same problem using winroute, after some research it turned out that the old version of winroute only allowed on outbound pptp and had to be upgraded.
So check with the provider of your internet gateway what has to be done to allow multiple pptp connections

Leopold