The cup rim sensor is soiled.
The cup rim sensor is located under the central spout. Clean the cup rim sensor at least once a week and more often if heavily soiled.
Proceed as follows to clean the cup rim sensor:
Press the two latches together, see figure. The cup rim sensor is released.
Clean the cup rim sensor in the dishwasher or by hand using warm water and a little washing-up liquid.
To refit the cup rim sensor, press it into the holder on the central spout from below.