Bosch dishwasher SGS5602 filling problems

A friend just drove 750km to bring me a secondhand Bosch dishwasher(thanks Frans:-). A novelty in the house! Once installed it seems to have problems filling up. All filters are cleaned and it doesn't look too calcified. It just doesn't reach the right level in the machine. The machine works perfectly if added water manually during the cycle.
I'm not an expert on dishwashers but I'm a good mechanics/electronics repairman.
So my question is if someone can explain me how this machine works an what can be the cause of the problem.
thanks in advance for your help, Derk-Jan
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