grub install failed fatal error

[fizallo]

I'm trying to install Ubuntu as dualboot to Windows 10. But every time, grub fails to install. At the end of the installation, there's the error "Grub installation failed. This is a fatal error. " I tried the following things:

    secure boot is disabled

    fast boot ist disabled

    win is installed in EFI mode so of course I'm installing Ubuntu in EFi mode too

Some more infos: It's a 64 Bit acer Laptop with UEFI, with an AMD Radeon E2-9000e and a Radeon R2 graphic card and 4GB RAM DDR4 if that matters. During the installation, I select something else, create a 245.7GB / partition, a 4.2GB swap partition and around 200MB for EFI boot. After the first time, the installation always says that ubuntu is already installed, I guess that means only the grub installation failed, but Ubuntu was installed. I googled a bit, but did not find anything that worked.

By the way, the installation USB is probably not a problem, just tested it on another PC and it worked instantly. I also tried both the "install Ubuntu" option and the "try Ubuntu" and then installing it.

Does anyone know how to fix this?

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: ubiquity 20.04.15.2
ProcVersionSignature: Ubuntu 5.4.0-42.46-generic 5.4.44
Uname: Linux 5.4.0-42-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu27.4
Architecture: amd64
CasperMD5CheckResult: pass
CasperVersion: 1.445.1
Date: Wed Oct 7 11:27:45 2020
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/xubuntu.seed only-ubiquity quiet splash nomodeset ---
LiveMediaBuild: Xubuntu 20.04.1 LTS "Focal Fossa" - Release amd64 (20200731)
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)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.