Could you advice me how to measure it?
I'm sorry - but this cannot be done remotely! The electrician has to inspect the whole premises.
I understand you but lights start flicker when click on switch (position on), but relay does not click. On SW is no voltage.
Is it possible to turn the lights on via the Shelly cloud or the WebUI? Are the lights still flickering even if they were switches by the WebUI or the cloud app? Are there any entries in event log? (Shelly Cloud)