Okay, here is the first test, following the schematic given.

I have used the Analog Discovery 2 and applied the probe as it is shown in the diagram:

Ignore the microcontroller for now.

The square wave is configured at 200Hz, offset 1V and 1V amplitude.

For the switch, it is substituted by the channel 0 in the Analog Discovery 2.

The channel 0 is selected as Switch->Push-Pull.

And the output: (the 100nF capacitor is placed before the R3 which is not shown there)

Flicking the switch up and down doesn't do any effect. Believe me, nothing at all! But how did I managed to get it to work earlier? My fault for not jotting this down, and I'm going to probably recreate it (with a different resistor combination), or possibly ditch this design permanently.