Earth decoupled power supply
Even if I connect the system ground to the mains protective earth, there is still a voltage up to 50V between mains protective earth and the body standing on the ground.


Then your mains protective earth is floating and is not bonded to an Earth Ground Electrode as it should be. The 50 VAC is a result of the capacitive coupling between HOT/GND and GND/NEUTRAL where GND is allowed to float.