In your photo of the unit and the DVM I cannot tell if the low voltage electronics is earthed or not. I can see a yellow wire that might be an earth, but based on what you say I will assume not.
The neutral wire will have 230VAC on it coming from the load while the relays is not operated. As adwsystems pointed out, wood is not a (very good) insulator, so you have 230VAC being applied to the wood at one end and you are measuring some of that at the Arduino USB connector at the other end.
Earth your low voltage electronics and put a good insulator under the board with an air gap. I usually use plastic sheet of similar thickness to window glass, available at DIY places. You can buy nylon nuts and bolts and spacers to mount everything.