vpn, pptp & linux router

Hi!

I have a problem with VPN connection.
2 comps are connecting to Linux router and then to VPN Server.
I don't have logs right now and i'm writing what i've remebered.

Kernel 2.4.19 and patch netfilter_pptp.
ip_conntrack_pptp and ip_nat_pptp are compiled as modules and loaded by
insmod. I've warrnings, but also message <module_name> Registered.

Firewall is allowing incoming packets from VPN server on protocol TCP port
1723 and on protocol GRE. All outgoing from LAN and of course masquerading.

Clients have Win XP. When 1st client establishes connection everything
works, but when second is starting connection i have a message:
failed to register conntrack protocol GRE (maybe it's already registered?)
and then first one is disconected, second works fine.

Everything works when 1st client is connected directly to Internet and
second client is connecting with first as default gateway.

Is connection possible by linux router?
tldp.org says yes, different sites say no.

It works the same with or without ip_conntrack_pptp and ip_nat_pptp. Only
logs are different...

I havn't installed patch to pptp server.
Maybe I have to? maybe another patch? maybe patch to iptables? I don't know.

It looks like router can't see clients CallID and I don't know why...

Of course I can connect every client to different IP, but that's not I've
wanted to do.

Sorry for my english and I'm waiting for some help..... thx