First upgrade with Combo went totally awry... I think I messed up with the copying of 'apfs.efi'.
Restored everything. Did not copy the downloaded 'apfs.efi' in /usr/standalone/i386/ this time.
Went for an upgrade via the App Store and Shazaam! (after the Nvidia-drivers update...
I had the same problem for the previous update.
What I did :
1) booted from an unibeast-installation key
2) selected then 'update Mac OSX'-drive (the one that appears when you have previously performed an upgrade)
After that, the installation went smoothly.
Hope this helps
Thanks a lot for a flawless update.
Copied new apfs.efi in 2 places:
Upgraded to Clover 4509
Used the Download the macOS High Sierra 10.13.5 Combo Update,
Updated the nVidia Drivers and ....