When defining a joint definition for a Proxy object, if Motion Type = Kinematics, an error occurs "The Kinematics joint does not belong to any opened chain", and the simulation cannot be run.If Motion Type = Dynamics or Articulation, an error does not occur.
It is designed behavior. The Kinematics chain must start from the ground. So when you create a fixed joint, the kinematics chain works. Kinematic objects only be driven by the motor, so you can't drag the mouse to move it. (in dynamics solver, drag mouse means apply force to an object to drive it to move)
Notes