setup Zyxel USG 20W as L2TP VPN Server

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http://serverfault.com – I've a Zywall USG 20W (wireless disabled) behind a router supplied by the ISP. All ports (both TCP and UDP) on the ISP router are forwarded to the 20W. I'm trying to configure an L2TP VPN to be used by Windows Xp / 7 with Microsoft native client. This was working before with a different firewall, so I'm pretty sure that all the required packets are flowing to the 20W. I followed a tutorial from the italian Zyxel Website, but I cannot get the VPN to work. Always cannot pass phase 2, and I see the following on the log: [ID]: Tunnel [Default_L2TP_VPN_Connection] Phase 2 local policy mismatch Phas (HowTos)