We are unable to instantiate the HSSFWorkbook-object.
HelloExcel class is executed with all poi-jars in our classpath as wel as the 
log4j.jar

statement in error:

    HSSFWorkbook werkboek = new HSSFWorkbook();

This is the error:
Message ID: RNQ0301
Message: Java exception received when calling Java method (C G D F).
cause: RPG procedure TRQTSRC in program TRQTSRC/LXCONT received Java 
exception "java.lang.NoSuchMethodError:" when calling method "<init>" with 
signature "()V" in class "net.sourceforge.poi.hssf.usermodel.HSSFWorkbook".
...

this is the code of HelloExcel:

import java.io.*;
import net.sourceforge.poi.hssf.usermodel.*;

public class HelloExcel {
  public static void main(String[] args)
    throws IOException {
    FileOutputStream bestand =
      new FileOutputStream(args[0]);
    HSSFWorkbook werkboek = new HSSFWorkbook();
    HSSFSheet werkblad = werkboek.createSheet();
    werkboek.setSheetName(0, "eerste werkblad");
    for (short rownum = (short) 0; rownum < 10; rownum++) {
      HSSFRow rij = werkblad.createRow(rownum);
      HSSFCell cel = rij.createCell(
        (short) 0, HSSFCell.CELL_TYPE_NUMERIC);
      cel.setCellValue(rownum + 1);
      cel = rij.createCell(
        (short) 1, HSSFCell.CELL_TYPE_NUMERIC);
      cel.setCellValue((rownum + 1) * (rownum + 1));
    }
    werkboek.write(bestand);
    bestand.close();
  }
}