http://bmwfans.info/parts-catalog/F2..._fuel_pump_k12
Can this be helpful?
I assume you have replaced aux water pump already. In my case, I had to change the aux water pump as well as the water pump relay.
Maybe the relay causes the same error again, its a cheap part (cca. 15€) so it is worth checking. It would be helpful if you write what errors appeared after few days?