grub-install failed fatal error when installing Ubuntu 20.10

1) Ubuntu release: 20.10
2) Package version: unknown (not available from ubiquity installer)
3) Expected : successful install. The default partition option to install Ubuntu besides Windows was chosen - single drive with 3 Windows partitions (Recovery, System Reserved and main C partition) and ample unallocated space left for Ubuntu.
4) What happened: following error "L'exécution de grub-install /dev/sda a échoué. Cette erreur est fatale" (rough translation: "Running grub-install /dev/sda failed. This is a fatal error").

ProblemType: Bug
DistroRelease: Ubuntu 20.10
Package: ubiquity 20.10.13
ProcVersionSignature: Ubuntu 5.8.0-25.26-generic 5.8.14
Uname: Linux 5.8.0-25-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu50
Architecture: amd64
CasperMD5CheckResult: pass
CasperVersion: 1.455
CurrentDesktop: ubuntu:GNOME
Date: Tue Dec 15 21:42:41 2020
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed maybe-ubiquity quiet splash ---
LiveMediaBuild: Ubuntu 20.10 "Groovy Gorilla" - Release amd64 (20201022)
ProcEnviron:
 LANGUAGE=fr_FR.UTF-8
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=fr_FR.UTF-8
 LC_NUMERIC=C.UTF-8
RebootRequiredPkgs:
 linux-image-5.8.0-25-generic
 linux-base
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)