What's a Zanussi washing machine E20 fault?

Originally I had a E40 fault and the machine would not start the spin cycle. I drained the machine using the emergency drain hose at the front then ran a rinse program but it stopped with 18 minutes to go and error code E20. I drained the machine manually again and then the machine did its spin.
Why isn't the machine discharging the water? I've checked the outlet to the drain and the filter and both are clear.