DIAGNOSIS Water is leaking from the tank's internal pump or seal area. The most likely cause is a tiny piece of debris (like limescale or sand) stuck in the pump's valve, preventing it from sealing.
FIX Remove the clean water tank. Empty it. Flip it over and look at the black pump mechanism on the bottom. There's a small rubber flap or seal. Use a soft toothbrush or your finger to gently wipe away any grit or buildup around that seal. Rinse the whole tank, shake it dry, and refill.
IF THAT DIDN'T WORK Inspect the silicone seal/gasket that runs around the top rim of the tank where it mates with the main unit. If it's pinched, torn, or has a hair stuck in it, it won't seal. You can often reseat it with your fingers.
HEADS UP Don't use hot water in the clean tank. It can warp the plastic and compromise the seals. Cool or lukewarm tap water only. Also, never store it with water in the tank on the dock; empty both tanks after use.