This message means that the ASA received an ICMP error message (in this case type 3, code 3--this is an ICMP port unreachable message) that it did not have a corresponding connection for.
Based on the information in the message, it sounds like you either have a client that is configured to query the wrong DNS server, or your DNS server is not listening on the correct port.
These messages are probably not cause for concern from a security standpoint, but they could indicate a configuration error. I would recommend setting up some packet captures on the ASA to look at the entire conversation and see exactly what is going on.
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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...