E20 error code?

I have an AEG Lavamat Turbo 16850 Washer Dryer, which occasionally displays an E20 error.

I have checked that the drain filter is perfectly clean and the pump impellor turns freely (which is always the case). I have also removed and thoroughly cleaned the Condenser. Once I manually drain the machine and start it up again it works fine for a few washes until the E20 comes up again and we find it full of water. I have added water through the door and put it through many drain sequences and all is well and then out of the blue a few washes later it stops on an E20 error again. I have proved time after time that the filter is clean and the pump works, so at a loss as to what is causing the problem.