This might seem too obvious but sometimes those errors appear in non custom things when trying to do global transitions. Have you tried opening a new scene and giving it another shot, or make a new game object set and try connecting it again?
Make the event you plan to make global and THEN try to connect it, see if that helps.
Sorry it's not great help, I'm just a customer. lol But say I set up a global transition for awsd; I've had the error fire on D while the rest were fine, as example.