The message is related to DOS attack prevention. It could be in response to an actual attack or in some cases due to routing problems or packet corruption. A similar problem that I know of was in a setup where some of the clients were behind a Linksys router and had something to do with the way in which the linksys routers handle IPSec tunneling. If you too have devices behind a Linksys, then the error messages are to be expected.
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...