High NG

I hop I'm in the right group. I try to establish a vpn connection to a
windows 2000 server with pptp ( high encryption). From the router ( with
fixed ip ) i have a port forwarding of 1723 and GRE to a ipcop and form the
ipcop i have a forwarding of these protocols to the server.
Everything works fine for a short time and than  - zap the connection is
disconnected. I can reconnect and work further for 3 minutes. I have
searched a lot in the last two days but found no solution.
I have attached the protocol ( only the last lines ) of the providers packet
sniffer.
Can you tell me what is going wrong here ? The log on the server says that
it disconnects for user demand.
What is port 2853 ?

Thank you for your time and help
Florian

Intergface explanation:

192.168.0.204 (ipcop extern)
213.23.130.186 (me from the internet )


Log
10/13/2005-13:59:41:IP-FILTER: I:PROTO 47 (???) pkt from 192.168.0.204

10/13/2005-13:59:41:IP-FILTER: to 213.23.130.186 accepted

10/13/2005-13:59:41:IP-FILTER: O:PROTO 47 (???) pkt from 213.23.130.186

10/13/2005-13:59:41:IP-FILTER: to 192.168.0.204 accepted

10/13/2005-13:59:44:IP-FILTER: O:PROTO 47 (???) pkt from 213.23.130.186

10/13/2005-13:59:44:IP-FILTER: to 192.168.0.204 accepted

10/13/2005-13:59:44:IP-FILTER: O:PROTO 47 (???) pkt from 213.23.130.186

10/13/2005-13:59:44:IP-FILTER: to 192.168.0.204 accepted

10/13/2005-13:59:45:IP-FILTER: O:PROTO 47 (???) pkt from 213.23.130.186

10/13/2005-13:59:45:IP-FILTER: to 192.168.0.204 accepted

10/13/2005-13:59:46:IP-FILTER: O:PROTO 47 (???) pkt from 213.23.130.186

10/13/2005-13:59:46:IP-FILTER: to 192.168.0.204 accepted

10/13/2005-13:59:46:IP-FILTER: O:PROTO 47 (???) pkt from 213.23.130.186

10/13/2005-13:59:46:IP-FILTER: to 192.168.0.204 accepted

10/13/2005-13:59:48:IP-FILTER: O:PROTO 47 (???) pkt from 213.23.130.186

10/13/2005-13:59:48:IP-FILTER: to 192.168.0.204 accepted

10/13/2005-13:59:52:IP-FILTER: O:PROTO 47 (???) pkt from 213.23.130.186

10/13/2005-13:59:52:IP-FILTER: to 192.168.0.204 accepted

10/13/2005-14:00:00:IP-FILTER: O:PROTO 6 (TCP) pkt from

213.23.130.186/2853

10/13/2005-14:00:00:IP-FILTER: to 192.168.0.204/1723 accepted, ACK

Flag

10/13/2005-14:00:00:IP-FILTER: I:PROTO 6 (TCP) pkt from

192.168.0.204/1723

10/13/2005-14:00:00:IP-FILTER: to 213.23.130.186/2853 accepted, No

SYN/ACK

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