We have a Vigor2860 setup with the L2TP/IPSEC VPN. I can connect to this using credentials configured on the router, however when I try to use an AD account via our Radius server connection fails. We are getting the ever helpful 'Error 734: The PPP link control protocol was terminated'.

I know the VPN will work as router based credentials work straight away.
The router is definitely hitting the RADIUS server as if I block it from the RADIUS server I can see access attempts in its logs.
Using a RADIUS test app I can authenticate against the RADIUS server.

Any ideas as what might be wrong or were to start!