Hi Ralf,

DOM elements have a value property (not a function), it should be:
customer_number = document.getElementById('customer__number').value;
It might be that you are confusing it with the getValue function of the AJAX Library:
customer_number = vdf.getForm("myform").getDEO("customer__number").getValue();
ps. Sorry for using English, I do read German, but writing is a completely different thing...