Hi there
I've had to periodically unblock our WD72 when it comes up with this message. I'm afraid I don;t know the correct terms, but taek the back off and there is black rubber hose that is fastened to the pump at one end and at the other end houses a collector of bits - buttons, socks (the main culprit) etc. usually it is this collector that is blocked, sometime sthe other end too.
Ironically I am on here now because this time I think it's the pump itself that has gone- just did a random search and found this and though I'd share what I've found out.
Nick Roscoe, October 2005