I had already installed one of the HS200 in another location and it works perfectly fine.

Now I put one in where the only wires coming in to the box are one white, and one black. The previous toggle switch works perfectly, when OFF the light is completely off.

However when the HS200 is off, the light is on, just dimmer than before. If I put a regular (non-dimmable) bulb in, the bulb turns on and off every half a second. And when this is happening, the switch is constantly losing and gaining power as if the bulb controls the supply. With no bulb, the switch has no power. I don't understand what could be happening as the previous dumb light switch and fixture work fine when I replace the original toggle switch.

I've tried both black wires from the switch to the one black in the box. I've tried just one black from switch and capping the other one. I've even tried reversing the connections (black to white and vice versa), same results

Exactly Same thing as was reported here - https://community.tp-link.com/en/home/forum/topic/151249

Thanks!