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