Hello Bruno,

I took a look at your PCB, hope you did not order it already as there is a big issue.
Please see red circle in attached picture. Distance between traces is less than 0.3mm! Absolute recommended minimum between mains and other signals is 3mm for 230V.
As soon as there is a connection or just significant capacitance to ground on the low Voltage part of your PCB (like human being or long wires to light switch) your arduino and rest will most likely immediately be destroyed

Also orange circle shows issues.. depending whats soldered to the left pads 1mm is not enough.. And be sure to keep enough distance between Arduino board and soldered pins of HLK-PM01.

Jantle

edit: of course also on top side of PCB traces are to close to pins of the relay...