For anyone interested:
It took me some time but now i can tell what happened:
I created an AnimatedSprite2D. It's empty because it's only there to be inherited by other scenes. Before the heridity i once added some Sprite Frames and a default animation. After implementing inherited scenes i deleted the "default" animation at the bottom SpriteFrames tab. But i forgot deleting SpriteFrames at the inspector.
Now after every start it flooded me with the error described above.
After clearing SpriteFrames at the inspector tab it gives me another error:
E 0:00:00:0377 set_animation: There is no animation with name ''.
For some reason he still tries to load some animation...
To resolve this simply once again click on "New Sprite Frames" and clear them again. Now it works.