This post is in relation to a Miele W1712, but may apply to other front loader machines.
During the wash and rinse fill, water was flowing out of the detergent dispenser. It was not obstructed, nor was the interior.
Water is dispensed from above through a series of jets set at angles to each other, washing the pre wash detergent, detergent and through separate jets conditioner out of the dispenser down to the rear of the bay into a rubber hose connected to the drum. I reasoned that there was a partial obstruction, with water back filling after a time. The pump filter was checked and cleaned - no help.
Disconnect power.
To open it up, remove the dispenser, remove 2 oval screw covers located on the tops outside edge near the front of the machine - find yours for other models. Back the screws out 6 or so turns but don't remove, this action pulls a catch away from its locking slot. Remove screws at the rear of the top. Lift the top from the front, to around 45 degrees then unhook the rear, lifting the top away. You can now see the tube. Tilt the machine up, wedging a support under it. Half fill a bucket. The next bit is easier with 2 people but 1 works. Poor water down the dispenser, quickly grab the tube and rapidly squeeze several times, repeat several times. Lower the machine.
Open up the pump drain, drain out the water, you should find slime, foreign object or whatever was blocking the inflow tube. Clean up the mess you have invariably made.
Replace dispenser. Power up and test. If no leaks, disconnect power reassemble unit. Hope this helps.