I have had similar problems. Mine were due to the drain pipe from the air vent at the top being blocked. There is a rubber tube that runs from the air vent down to an area beside the soap dispenser, and this blocked witha fatty substance. The tube just pulls off for cleaning. In the end I put it down to the eco detergent we used, and changed to ordinary Waitrose detergent. It hasn't recurred, but that may be because we haven't left it long enough. An occasional very hot cycle would help (in fact might even cure it). To get at the works there are 6 screws on the sides of the inside of the door, accessible with the door open
Andy, November 2008