Dishwasher not filling with water?

My AEG FAVORIT model F77012WOP does not fill with water at the beginning of the washing cycle. It does take on water during the rinse, and empties during the rinse, but no water goes in at the beginning of the washing cycle.

The machine starts and I can hear a noise, which I assume is the pump running, but after a few minutes it displays the code 20! When I open the door you can see that no water was taken in.

I have replaced the control board, as recommended by the service technician that "diagnosed" the fault, but after replacing the main control board it still does not work.

I would be grateful for any ideas on what the fault might be and how to fix it.

Thanks