K7201 Unstable display

Years ago I’ve bought a Velleman power supply kit, the K7202 with the K7201 as display. A few weeks ago I found it again and fixed it to working condition. The only thing that’s bothering me is that the display is unstable. When I set it to 5V the display is range from 4,5V to 5,5 V. When I was searching the internet about this problem I found the product page from Velleman about the display with a “Hint” section, that’s stated the following:

  • When display is unstable ground front panel.

Is the front panel, the panel of the power supply or the pcb?

Can someone tell me?