18.04LTS cannot be installed: failed to install grub

[joeridoe]

1. 16.04LTS was running fine.
2. Tried to upgrade via the software&Updates of Ubuntu towards 18.04LTS: this failed with a lot of unmet dependencies, which i was not able to resolve.
3. Decided to install 18.04LTS from scratch: installed the iso file on an external SSD.
4. 18.04LTS installation detected my broken 18.04LTS on the disk, and suggested to reinstall it.
At the end of the process, i got an error : bootloader could not be installed.
5. Did the 18.04LTS installation a 2nd time, and chose the option to erase the full disk, and install 18.04LTS on it. Again the same failure at the end: could not install bootloader

In both case 4 and 5, i got a window : please chose another location to write the bootloader to.
When i selected the same disk again, i got the same error.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubiquity 18.04.14.14
ProcVersionSignature: Ubuntu 5.3.0-28.30~18.04.1-generic 5.3.13
Uname: Linux 5.3.0-28-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.9
Architecture: amd64
CasperVersion: 1.394.3
CurrentDesktop: ubuntu:GNOME
Date: Wed Jul 15 18:00:11 2020
InstallCmdLine: file=/cdrom/preseed/ubuntu.seed boot=casper initrd=/casper/initrd quiet splash --- maybe-ubiquity
LiveMediaBuild: Ubuntu 18.04.4 LTS "Bionic Beaver" - Release amd64 (20200203.1)
ProcEnviron:
 LANGUAGE=en_US.UTF-8
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 LC_NUMERIC=C.UTF-8
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)