Soiling and/or coffee grounds, both of which can cause mould growth, have accumulated behind the brew unit inside the coffee machine.
Clean the brew unit and the inside of the coffee machine once a week. Proceed as follows:
Disconnect the coffee machine from the power supply before cleaning.
Open the appliance door on the right-hand side of the coffee machine.
Press the button on the bottom of the brew unit handle (1). Whilst doing so, turn the handle to the left (2).
Carefully pull the brew unit out of the coffee machine.
Clean the brew unit by hand under hot running water without using any cleaning agent. Rub away any coffee residues from the filters.
Using a vacuum cleaner or standard brush, remove any soiling or accumulated coffee grounds from the inside of the coffee machine.
To reinsert the brew unit, repeat these steps in the reverse order:
Slide the brew unit back into the coffee machine, ensuring it remains straight.
Press the button on the bottom of the brew unit handle (1). Whilst doing so, turn the handle to the right (2).
Close the appliance door.