@VLC,
It could happen due to datatype difference in column and 600.
Use assign activity to filter it using LINQ like below.
filteredDataTable = (From row In inputDataTable.AsEnumerable()
Where IsNumeric(row("Nettowert").ToString()) AndAlso Convert.ToDouble(row("Nettowert").ToString().Replace(",", ".")) < 600
Select row).CopyToDataTable()