Hi there, just wanted to give an update.
We have a fix for REINST_ errors coming soon.
To clarify, the method of communicating between tasks, services and decorators should be to use your black board as described by dbc211. Since the original question was posted, we now offer a complete quickstart guide that will help to clarify what the best practices are with behavior trees. Here is the guide if you are interested:
https://docs.unrealengine.com/latest/INT/Engine/AI/BehaviorTrees/QuickStart/