Authentication should never work between an unknown NAS and the AAA server.
Further Authentication should never work if the shared key is also different.
If Authentication is working after the IP of the switch has changed, then it would suggests one of two things
1) You have a wildcard address range defined that has the same shared key as your switch has
2) A bug in ACS
However judging by your description of the logs it implies that it is neither of these since it is reporting correctly that the NAS is unknown.
If users are still gaining access and then it suggests to me that the switch has some kind of fallback mode that if AAA functionality is not working allow users some kind of locally provisioned access.
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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...