Comment 13 for bug 1924823

Got the WORKAROUND and it's quite easy :D
(Thanks guiverc, you gave the idea!)

As you have done I tried to boot the system keeping the USB installer device connected to the computer. I've just set 'first boot from hard disk' inside bios settings... and the system boots.
Then I've uninstalled grub-efi* package, modified /etc/fstab and re-installed grub-pc. It worked!

HERE ALL THE STEPS:

1. Make sure 'first boot from hard disk' is set in the BIOS settings.
2. Connect the USB installer device and start the computer.

3. Open /etc/fstab and comment the line related to /boot/efi:
sudo -H gedit /etc/fstab

4. find a line like this:
UUID=BF7E1-08E7 /boot/efi vfat umask=0077 0 1

5. put the character # at the beginning of the line:
#UUID=BF7E1-08E7 /boot/efi vfat umask=0077 0 1

6. Save the file and exit from gedit.

7. Un-mount e mount the partitions:
sudo umount -a && sudo mount -a

8. Uninstall all grub-efi related packages:
sudo apt purge grub-efi*

9. Reinstall grub-pc:
sudo apt install --reinstall grub-pc