With this post I would like to say thanks to the documentation team. I made the boot manager useless by applying a change via systemd and not via clr-boot-manager of which I ignored existence. Result: no boot. I followed this tutorial by including 5 seconds display:
https://help.getsol.us/docs/user/troubleshooting/boot-rescue/system-not-working-hardware-change
Despite a clr-boot-manager error (did not know where to write) at the end of the procedure I could make the reboot successfully.
Thanks a lot to the solus team