Generally this happens if the site from which you are connecting uses the same subnet locally on the VPN client's LAN as the Draytek's LAN, such as both using 192.168.1.x If this is the case you will either be able to connect but not access any resources, or if the "use remote default gateway" option is selected you will be able to connect and only access the VPN server (Draytek) itself.
If this is the case the only solution is to change the subnet (not subnet mask) at one site or the other. This is why at the Draytek site common subnets such as the following should be avoided:
192.168.0.x, 192.168.1.x, 192.168.2.x
10.0.0.x, 10.10.10.x, 10.1.1.x
172.16.0.x
For the record the Gateway option can be accessed by: On the VPN client machine right click on the VPN/PPP adapter in network connections and choose properties, then under Networking | Internet protocol IPv4 properties | advanced | check the box "use default gateway on remote network"
Based on your description I suspect the problem is the above, but you can also run into this issue if you have the Draytek assign the VPN PPP client an IP in a different subnet than the Draytek LAN. In this case the client doesn't have the correct routing information. For example:
Draytek LAN = 192.168.100.x
VPN client's LAN = 192.168.1.x
VPN client PPP adapter's current dynamic IP 192.168.200.101
The VPN client in this situation has routing information to connect to the 192.168.200.x network via it's VPN/PPP adapter but no idea how to connect to the 192.168.100.x LAN. In this case the client needs to have the following route added:
route add 192.168.100.0 mask 192.168.200.101
