Selim thanks, it turned out to be both at once. I removed the jumper and connected a separate 5V adapter, and fitted an RC snubber module next to the coil. For a week now the relay has been clicking on and off every evening, and there is no single keepalive_timeout on the Log tab. I never really understood what JD-VCC is for; I thought there was isolation because the board has optocouplers. I also applied the auxiliary contact idea, and now the panel shows whether the pump is actually running.