Hello again,

That is very weird. I am also on Unity 5.5 and tested exactly what you said. Having a task that references a global bb variable, saved the scene, closed unity, reopened it and loaded the scene. I get no errors at all.
OnValidate() is not really called during serialization as far as I know, otherwise there would be much more problems that this one in all of my projects 🙂

I’m sorry, but I really can’t reproduce the error you have here :/