Outgoing L2TP VPN doesn't work through one of two LANs on same router

We have a Draytek 2960 router running the latest firmware (  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.
David HaycoxAsked:
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David HaycoxAuthor Commented:
The firmware update didn't make any difference, however I traced the problem to a setting on a switch - an HP Procurve HP 1810-48G (J9660).  Disabling "Auto DoS" fixed it.
TimotiStDatacenter TechnicianCommented:
Looks like Draytek's have some issues with L2TP.
The latest-greatest 1.0.8 firmware is out for the 2960, it mentions some L2TP fixes, you could try your luck with it.

David HaycoxAuthor Commented:
Thanks, will give it a try.  Only just upgraded it to - good to see the router is still under active development though.
David HaycoxAuthor Commented:
Found solution myself.
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