I own a Gaggia Syncrony Compact machine. My water tank is leaking at the valve. It appears to operate fine while in the machine. When I remove the tank to refill at the sink there is a slow drip from the valve.
First try descaling the machine, because limescale could block the valve from closing all the way.. If that does not work you would need to replace the o-ring on the water tank itself.
Oring Specs - EPDM 0019-10