but the LEDs stay quite. So main loop is not called any more, the system is dead.
No, it's just not doing what you want it to do, there's a huge difference.
It's not really practical to debug a problem like this with LEDs, you should have proper equipment. You have to know where the code gets to, staring at non-flashing LEDs is no help.
That said you could set the LED in the ISR instead of the PULSE macro I suggested. That will at least tell you what part of the ISR (if any) you execute.
Rob