Here’s a solution provided by DarkHorror in the forum thread linked in my question:
The problem is created when trying to
access other objects and object
variables in your Task or other
behavior tree type blueprints. All you
need to do to fix that is simply add
those objects to your blackboard and
access them through the blackboard
rather than trying to do it directly.
It doesn’t seem like this should be necessary, but it’s a good workaround for now!