[vpn-help] V2.17 connect to AVM Fritz!Box problem, IKE response not recognized

[Christian Haul]

Hi,
I have a working setup with an AVM Fritz!Box and a Debian Stable aka
"Squeeze" (Shrew IKED 2.1.5). However, I cannot get the same to work on
a Debian "Sid" aka Unstable (please bear with me, Shrew IKED 2.1.7).

According to the logs, the IKE phase 1 message is sent:

11/08/04 19:52:32 -> : send IKE packet 192.168.2.101:500 ->
188.xxx.xxx.xxx:500 ( 521 bytes )
11/08/04 19:52:32 DB : phase1 resend event scheduled ( ref count = 2 )
11/08/04 19:52:32 ii : opened tap device tap0

Looking at
        pcap_decrypt "/var/log/ike-decrypt.pcap";
        pcap_encrypt "/var/log/ike-encrypt.pcap";

pcap_decrypt shows nothing (0 bytes)
pcap_encrypt shows only the message in the log.

Capturing simultaneously with wireshark shows a response, however.

NATT is enabled, port is 4500. Wireshark shows port 500 for source of
outgoing and destination of incoming packet.

lsof shows iked.real is listening on udp:500 and udp:4500

Any idea what goes wrong here? Or how I could investigate further?
Will next wireshark the working machine.

TIA, Chris.