@absolut : As I said in my last post the problem occurs at the start of systemd, there is no journaling at this point. If you prefer the error occurs with the start of PID=1. When I chroot into the unbootable system, the last item on the journal concerns the last working boot.
@der_bud : I find no way to start systemd, so no use of systemctl or journalctl.
The only way would be to be able to read those messages during boot to know exactly which services are inside this cycle. but as I explained earlier it goes too fast and I don't know the way to record the boot log before the start of systemd.
Any body have an idea ?