I suppose that the PIX1 has a NAT on the router. I also suppose you've already tried to ping between the PIXs to make sure that they can talk to each other in the first place.
You mentioned that you've allowed protocol 50 & protocol 51. Have you allowed UDP/500 port also? Important to IPSec to work. If you've opened UDP/500 and the PROTOCOLS (note that they are not port numbers) then post your PIX configurations and the router configuration as well.
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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...