Usually the impeller comes apart in two pieces. The dishwasher detects low pump pressure, stops the wash, drains the water and ends the wash programme with F78 error. You need to glue the pump impeller with special industrial adhesive and leave to set for 24 hours, you also need to take it apart, don't glue it in place. Made a video about it, search youtube for f78 miele impeller diy. Miele wants 400eur for a new full pump, for older models there is a repair kit.