You should still be specifying the sheet for all your ranges:
Sub KOLBOPLO_1()
    With ActiveSheet.Sort
        .SortFields.Clear
        .SortFields.Add Key:=ActiveSheet.Range("B6:B19"), SortOn:=xlSortOnValues, Order:=xlAscending, DataOption:=xlSortNormal
        .SortFields.Add Key:=ActiveSheet.Range("C6:C19"), SortOn:=xlSortOnValues, Order:=xlDescending, DataOption:=xlSortNormal
        .SetRange ActiveSheet.Range("A6:C19")
        .Header = xlNo
        .MatchCase = False
        .Orientation = xlTopToBottom
        .SortMethod = xlPinYin
        .Apply
    End With
End Sub