I've had the same problem on my WD72 - it wouldn't empty and reported E15.
I followed Nick's advice (thank you) but the filter was clear. It was only after I took the pump out (with the intention of replacing it) that I found a shoe lace wrapped around the pump's blades.
My tip - roll the machine onto its front to remove the pump. The 2 bolts underneath it are much easier to undo than the three fiddly allen key bolts on the blindside of the pump. Once the pump's out, these final three bolts come off easily to expose the blades.
Ben
August 2006