IllegalStateException, but it works on J2ME WTK2! :(

Question

I build and package the project...it uses CLDC 1.0 and MIDP1 (so no compatibility problems with phones), I execute the jad file on the PC and everything works. But I have tried it in 2 phones (a Sony Ericsson T630 and a Nokia 6100, both have MIDP 1, Installation worked in both) and I just get the IllegalStateException message.
It's very simple, no images used, one class, just "high level GUI".

Answer 1

Arg, no edit option in this forum ;( The program just has this methods and builder:
import java.io.*;
import javax.microedition.midlet.*;
import javax.microedition.rms.*;
import javax.microedition.lcdui.*;
public class Preventa extends MIDlet implements CommandListener{
//create some vars (record store, display...)
public Preventa(){
//initate and create vars
public void startApp() {
//some code, ended with pantalla.setCurrent(hola,listacli); (display pantalla )
public void pauseApp() {
public void destroyApp(boolean unconditional) {
public void commandAction(Command c, Displayable d){
//more code
Methods for opening, reading and writing RecordStores.
That's all! :(