Without seeing the configs or the actual src and dst IP addresses, it's a little hard to figure out, but what it looks like is the remote PIX is trying to initiate a tunnel negotiation with this PIX. The first ISAKMP packet comes in, this PIX checks it, agrees on the parameters of the negotiation, then sends an ISAKMP packet back to the remote PIX. Nothing happens after this and a short time later, this PIX retransmits the packet. Again it gets nothing back, and it retransmits again.
What's possibly happening is that the ISAKMP packets from this PIX are not reaching the other one. Is the other one the one behind the 2600? Make sure it is allowing UDP port 500 packets through. Make sure the remote PIX has the "sysopt connection permit-ipsec" command in it. Can you run the debugs on the remote PIX and see if it loks like it's receiving any ISAKMP packets? Are you doing NAT on the 2600, and if so, do you have a static translation set up for the outside address of the PIX?
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...