Zanussi 1221 error?

When I try to use the washing machine, I put the clothes in, select the cycle etc.. then....

The error light flashes...

If I open the door at the bottom, and let the excess water out from the previous cycle then start again, it works fine.

But this is happening every couple of cycles now and is becoming very annoying!..

I'm not sure whether its down to the pump failing and not getting all the water out?, or whether its down to limescale?.. or even something else?..

can anyone help?

Thanks in advance