Thank you for confirming, we got everything working after starting the update from scratch with better understanding.
Sorry for the bug report, my bad!
We're unable to open our ExternalBehaviors, even on an empty project. Double clicking one of them throws the following exception:
Exception thrown while invoking [OnOpenAssetAttribute] method 'BehaviorDesigner.Editor.ExternalBehaviorInspector:ClickAction (int,int)' : MissingMethodException...