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
Phase 1 goes fine. In Windows the error is always 788. This happens regardless the proposals I set in the phase 1 and 2 setting.
What should I check ? Is there any way to get more detailed diagnostic info (policy mismatch is too generic) ?

Thanks a lot to whom may help.

Massimo.