[vpn-help] Shrew 2.5.1 and Draytek Vigor 2710 VPN

[Karnin]

Dear list members,
that's my novice report to vpn-help list, so please be patient with me
:)

## Client:
Windows XP Sp3
Shrew VPN client version = 2.1.5
Fritz!Box 7170 with DYNDNS.ORG-dynamic IP, ADSL
with Firmware 29.04.80
Client-subnet 192.168.112.0

## Host:
Draytek Vigor 2710 with DYNDNS.ORG-dynamic IP, ADSL
with Firmware 3.3.5 Standard for AnnexB
Host-subnet 192.168.215.0

## Setup according to:
http://draytek.de/Beispiele/VPN/ShrewSoft_Client.pdf

## Problem:
esp-AES-Tunnel/Auth will be established without any problem.
Draytek Router/Gateway 192.168.215.1 at host side answers to ping,
router's admin-page can be accessed by IP adress => working.
A Synology diskstation DS207 (linux system!) 192.168.215.100 can be
pinged and accessed at host side => working.
BUT:
Not pingable or accessible are the windows machines in subnet
192.168.215.0:
-Windows 2003 Server Standard
-Windows XP Sp3
-Windows 98SE (only terminalclient)

Firewalls are switched off for exploration purposes.

Problem is reproducable on different Windows XP Sp3-clients (Subnet
192.168.112.0).

Really mysterious......

Any hint for us?

Thx and best regards, 
Christoph