new installation of xubuntu fails at installing grub-efi-amd64-signed

[lamed1]

reason for reinstalling was however that grub didnt work (went to minimal grub), so maybe the problem isnt with the new installation...

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
Date: Thu Mar 19 15:53:22 2020
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/xubuntu.seed boot=casper only-ubiquity quiet splash ---
LiveMediaBuild: Xubuntu 18.04.4 LTS "Bionic Beaver" - Release amd64 (20200203.1)
ProcEnviron:
 LANGUAGE=de_CH.UTF-8
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_CH.UTF-8
 LC_NUMERIC=C.UTF-8
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)