@DDON It's very odd that the switch will light up only when Neutral, Ground and the hot wire is connected to the load side. The fact that the switch doesn't do anything when Neutral, Ground, Line and...

@DDON Leave all the wires disconnected when testing to ground. This way no part of the switch is in equation and only the wires coming out of the wall. By removing ALL the bulbs you are essentially...

@DDON Lets take a step back. It sounds as if you have a multimeter you can use to verify the wires inside the box? With the black probe to to your ground, confirm the only other wire that shows...

@DDON This does sound lke a wiring issue. It sounds as if the existing switching may be on the neutral side of the circuit. With the hot wire going to the light(s) first. This can cause all sorts of...