I ran into a very similar issue as https://jira.ixsystems.com/browse/NAS-109512 after upgrading from 11.3-U4.1 to 12.0-U2.1.
It looks like the triggering event is a replication task. Where I have 2 systems running TrueNAS 12.0-U2.1. The source server is running fine without issue but the target server (this server) crashes every night when the replication task starts. Please see the attached screenshot with task settings. NOTE: I have two tasks nas2_0211 and nas2_0212. Both are configured the same and run on the same schedule.