The only way out of this stuck state is to remove the battery. After more trials, it goes better in the sense that Nav4all only crashes after 1 minute now ;-)
Interesting that a midlet can crash severely my i600
The IBM J9 with the same midlet does not crash the i600 but the midlet does not interact then with my keyboard <_<
Opened a case with Nav4all a week ago and no replies... Free software so I cannot really complain.