We have a Draytek 2960 router running the latest firmware (22.214.171.124). We have a remote site that we connect to using an L2TP VPN. This works fine from elsewhere (3G, home, someone else's Wi-Fi, etc.) and from our guest network. It won't work though from our main corporate network (we see error 789). Switch back to 3G or the guest network and we're on.
The guest network has DHCP from the router, and a firewall rule blocking access to our other internal networks. The corporate network has DHCP from the SBS2011 server, and unrestricted outgoing access in the firewall.
What setting should I change in order to rectify this? Thanks in advance for any suggestions.