Ok, problem is solved. It had just a stupid reason.
As we have more than 1.000 articles in the meantime, the desired raw material was simply not loaded into the list, because an option in the product-modul (Start => options => moduls and apps => products => section “parameter” => option "max number of products in dropdown list) was set to 1.000. After havin altered this to 0 the raw material was shown in the list…
Sometimes it is soooo easy… 
regards
Stefan
