How to mirror a composition? I have a moving solid mask composition, which forms a white line that moves vertically from the middle to the left and back.

I now want to create a copy of that composition, mirror it and link the position. So that the mirrored comp will move from middle to right and back.

How could I achieve this? Setting the width = -100% did not work.
Pre comp your layer, then apply the Mirror effect to the comp.  If your comps are 1920x1080, set the mirror centre to 960x540, and the reflection angle to 0.

I think your scaling to -100% didn't work because it's a masked layer.  You might have more luck using a shape layer to create your line, then you should be able to scale it as you suggested.