External power supply is drained when using USB

Hi. I have a strange problem.

I'm using my arduino to drive motor through a L298N motor driver board. The motor driver is powered from a 0-14V power supply I built at school many many years ago, set to 12V.
In order to make it work, I had to connect the GND-port on the driver to GND on the arduino.

When powering the arduino through a 9V battery pack, it works fine. However, if the arduino is connected to USB (for troubleshooting over the serial monitor), it seems to drain the external power supply.
If I turn on the power supply, it quickly builds up to 12V, but as soon as I connect the arduino to USB, it drops to ~2V.

My arduino board is a funduino.
Connections:
Digital 2/3 - optical sensor
Digital 5/6 - IN1/IN2 on the L298N board
Digital 11 - ENA on L298N
+5V/GND1 - optical sensor
GND2 - GND on L298N

I don't understand how this drains the external power supply, as the only thing connected between power supply and arduino is GND - GND.

Hard to say, but when the Arduino is powered by USB, there are actually two connections between the circuits, your "GND-GND" connection and also a connection through the electrical outlet ground wires, so there's a ground loop. With the "GND-GND" connection open, I wonder what you'd get if you measured the voltage between the power supply ground to the "earth" ground, also between the USB ground and earth ground.

Thank you for the hint.

When measuring, I found that my power supply is actually not +12V/0V on GND but 0V/-12V (I get -12V when measuring PSU GND -> USB GND and PSU GND -> battery GND. Strangely though, it still works fine when using battery)
Is there any way to fix this? Or would I have to completely change the power supply?