Cisco site-to-site VPN behind non cisco VDSL modem
The 876 initiating would work since it's initiating. It sounds like you have the peer IP address on the 1841 pointing to the modem the 876 is plugged into. If the modem holds the public IP, it's not going to be able to terminate the VPN session from the 1841. Try enabling bridge mode so that the 876 gets a public IP and then re-initiate from the 1841.
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...