Conclusion on this one : I had problems with my SD card backup and for some reason it did not re-install (raimbow screen), problem with boot partition… Tough day but doing the following finally worked - just in case it could be of interest for someone else:
- Flash a new SD Card with the backup
- Get a Raspbian Jessie install package - find the one the closest to the date of the files i saw on my boot partition (in my case, Sept 2017…)
- Copy the files from the boot partition of the just above downloaded file to the boot folder on the SD Card (except the text ones) from my Windows machine
- Finally, get Pi to boot
- Remove the zulu *.deb files from archives
- apt remove zulu
- apt install zulu
- and I’m back to where I was 10 days ago…
Thank you again @sihui & @Bruce_Osborne for your help and patience.