The grounds container is dirty.
Open the coffee machine door.
Lift the grounds container slightly and remove it by pulling it to the front out of the coffee machine.
Empty and clean the grounds container in the dishwasher or by hand using warm water and a little washing-up liquid.
Dry the grounds container.
Use a damp cloth to clean the insertion slot for the grounds container in the coffee machine.
Dry the insertion slot.
Push the grounds container back into the coffee machine and close the appliance door.