Update:
Fixed it by setting Static Mesh Component named SM_Tile to MOVEABLE in che Child:
SM_Schleuse_Waterplane was Moveable the whole time.
What is still weird: On the Parent, SM_Tile is set to STATIC an SM_Schleuse_Waterplane is set to MOVEABLE the whole Time and it was an is still working!
Is this my fault? Or is this an Engine bug?