DIAGNOSIS: Coffee leaking from the bottom of the machine, not the spout, usually points to one of two seals failing: the rubber gasket on the brewing head or the seal where the water tank connects.
FIX: First, check the big black silicone seal around the brewing chamber (lift the lever to see it). Coffee oils and tiny grounds build up there, hardening the seal and causing leaks. Unplug the machine, let it cool, then use a damp cloth to wipe that seal thoroughly. Manually open and close the head a few times to work it loose. Run a water-only cleaning cycle (three quick button presses) afterward to flush any loosened gunk.
IF THAT DIDN'T WORK: The leak might be from the water tank connection. Remove the tank and inspect the white silicone valve on the bottom of the tank and the corresponding receiver on the machine. Make sure there are no cracks or debris. A leaking tank usually means the tank itself needs replacement.
HEADS UP: Never use vinegar or harsh chemicals to clean the brewing head seal. It degrades the silicone. Use only water or a damp cloth. If the seal is visibly cracked, warped, or permanently flattened, you need a replacement gasket kit from Nespresso.