Oracle 10g NVL() not working?
Apparently this sub-query does not always return a value:

(Select OS_PRZU.ZUM2 From OS_PRZU
Where OS_PRZU.IDNR = GLAS_DATEN_BASIS.IDNR And OS_PRZU.KUNR = 999999)


meaning that the default customer probably does not have a price for some of the products.

Replace the sub-query with a hard-coded value in order to verify that NVL itself works as expected.