DIAGNOSIS The O-ring seal on the water reservoir's outlet port is cracked or displaced. Water bypasses it and drips inside the chassis.
FIX Remove the water reservoir. Look at the black rubber O-ring seated in the groove on the plastic outlet nipple. Clean the area. If the O-ring is cracked or flattened, replace it. A generic #10 O-ring from a hardware store often works. Lube it with a tiny bit of food-grade silicone grease before reseating.
IF THAT DIDN'T WORK The leak might be from the boiler assembly's gasket. You'll need to disassemble the machine to inspect the seal between the aluminum boiler and the plastic housing. This is a more involved repair.
HEADS UP Do not run the machine with this leak. Water inside the chassis can short the control board or heating element, causing a more expensive failure or a shock hazard.