@flo-feld Hey again. I forgot to mention or quote you on my reply just above, so I am doing it now. 
While the things I’ve state above hold true, I’ve also found a solution from someone else that was facing the same bug .
I’ve made the required changes in the context of Slate and have attached for you the changes in a unitypackage. I did test it and seems to work after saving the scene now. Please note though that this is still a hack around the Unity bug and thus the warning “Animator is not playing an AnimatorController” will still show, but only once this time at least.
Thank you.
4899599–473891–MecanimBaseClip.unitypackage (955 Bytes)