The grounds container is full.
Follow this step-by-step guide:
Danger of scalding
If you make a number of milk-based beverages one after the other, the metal cup tray (A19) becomes hot. Wait for it to cool down before touching it and handle it from the front only.
When removing the drip tray, the grounds container must be emptied, even if it contains few grounds. If this is not done, when you make the next coffees, the grounds container may fill up more than expected and clog the machine.
This can be done using the spatula on the brush provided.