What's the IDSM2 operation mode - inline or promiscuous?
Not sure if this is rerlated, and depends on your settings. With inline, traffic may or may not allowed to pass through without mandatory IDSM2 inspection. And if you removed the blade, this could have disconnect the traffic flow (L3 plus other SupEngine processing of the passing traffic). With this, the active SupEngine was unable to communicate and allow passing traffic to get throuigh. This cut-off is making the module isolated, and this could be the reason why the standby SupEngine triggered.
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...