Switching PSU problem

[The Master]

I've attached some pictures which I hope will help. The earth pin on the IEC socket has 2 green wires coming from it. (I've attached an old picture so you can see what's going on behind the PCB. There was only 1 wire back then.)
One wire goes to the PSU and then the front panel. The other goes to the screws holding the IEC socket then splits to the 10 metal sockets on the back and the ethernet module (little PCB top-left).
All 3 screws on the ethernet PCB and the 2 screws on the front panel are connected to PCB ground.

The 12V supply goes into the PCB at the back. The negative side connects to this PCBs ground plane. That ground plane is then connected to the ground plane on the ethernet module through the short wires between them (Red: 7V out, Black: ground, Green/Blue: data).

The ribbon cable to the front panel links the ground planes on the ethernet module and the front panel. It also has 5V/3.3V supplies and some data lines.

So all 3 PCBs have their ground planes connected but the ethernet module and front panel also have their ground planes connected to mains earth (and the PSU case). The PCB at the back is the only one without a separate earth connection as it's only contact with the outside world are the 4 pins on each mini-XLR socket. It doesn't even make contact with the metal cases of these sockets.

While the PCB was resetting I connected the multimeter to the 12V screw terminals on the PSU. This is what stops the problem until I take the probs off.
The high voltage was measured with 1 probe touching the top of the PSU case and the other touching the 3 screws on the ethernet module. The screw where the earth wire connects has the lowest voltage. I also put the probe on the back PCB where the ground wire connects to it from the ethernet module and this had the highest voltage.

There were 4 of these that were all turned on several months ago. The other 3 are still running fine now. This one was sat on top of 1 of the others so both would have experienced the same ambient temperatures and electrical noise etc.
There are another 4 of these which are still running after a year.

I opened up every PSU before powering up for the first time to check for solder balls, bad joints, lose components etc. Can never be too careful with cheap Chinese electronics

I'm trying to source another multimeter to double check the reading.