It's a water flow issue, and it usually happens in coffee pot mode or when making lots of water-based drinks one after the other when one part of the system can't keep up with water demand. Before calling out a technician suggest doing not one but two descales, as this will at least ensure that the water path is scale-free - if there's still a problem then the Miele tech will identify it when they come out. Or just put up with it if you can't face the cost!