Can't connect to windows xp vpn server from ipad

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http://serverfault.com – I have configured a Windows XP VPN Server on my laptop as described here. When I try to connect through other PC in my LAN, everything works correct - I can connect and use Internet through encrypted VPN. However, the iPad cannot connect - cannot connect to the server. However, the iPad sees that server. What can I do? I have opened ports 1723, 50, 51 and 500 on my router and my firewall is off. (HowTos)