I believe these messages indicate that there is no NAT between the two VPN peers. This does not highlight a NAT problem.
Perhaps you could provide more details, including the remainder of your debug output.
My own experience is when two different parties manage two different VPN endpoint devices it is worthwhile to meticulously review all the VPN settings to ensure that they match. Just a single small discrepancy could result in a failure to tunnel.
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...