same problem here, i opened the fairphone, removed all the dust and cleaned everything, the power button is not in fault, the switch connected on the motherboard doesn’t work either.
i’ve contacted the fairphone support, who told me they have no spare parts for motherboards for the moment, and advised to contact a repair shop (that might be a welding issue…)
do anyone know about a repair shop in Paris ? (Pierre, it seems you do ? 
)
thanks!