Hi - I have a Diplomat ADP8224 and had the same problem this week (4/12). There is an article here about this problem:
http://www.ukwhitegoods.co.uk/modules.php?name=Content&pa=showpage&pid=102
I've had an engineer out in the past when it was under warranty and he changed the white matrix both times to fix this.
I basically did what the article said, dragged a garden hose into the kitchen and put some high pressure water through the white matrix at the bottom of the dishwasher. If you hold the hose there, dirty water eventually starts coming out of some vent on the side. I tried starting the programme a couple of times after this. It didn't work first time, but I obviously did something right because it is now working OK. I finished off with a strong programme, one I don't normally use, to flush it out a bit more.
Hope this helps.
Tony Smith
December 2006