1 - Make sure you're mapped into an IP range that's in the same subnet as your remote network
2 - Make sure you don't collide with any DHCP-assigned IP addresses in that subnet - make sure you map into an IP address that's outside the DHCP assignment range
3 - In OSX Server VPN settings, have you set the client DNS to be your local gateway?
4 - Make sure your current local subnet and remote subnet aren't the same, i.e. if you're in network A and trying to VPN to network B, A and B can't have the same subnet mask
5 - Engineer's solution - turn VPN service off and on again...