package grub-pc 2.02-2ubuntu8.6 failed to install/upgrade: installed grub-pc package post-installation script subprocess returned error exit status 1

The update of grub2 failed. I don't understand why.

ProblemType: Package
DistroRelease: Ubuntu 18.04
Package: grub-pc 2.02-2ubuntu8.6
ProcVersionSignature: Ubuntu 4.15.0-36.39-generic 4.15.18
Uname: Linux 4.15.0-36-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.4
Architecture: amd64
Date: Sun Oct 14 19:58:16 2018
ErrorMessage: installed grub-pc package post-installation script subprocess returned error exit status 1
InstallationDate: Installed on 2018-09-26 (18 days ago)
InstallationMedia: Xubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
ProcCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.15.0-36-generic root=UUID=26afe02b-ea6c-49ae-b232-66dfbe6ef2e0 ro quiet splash vt.handoff=1
Python3Details: /usr/bin/python3.6, Python 3.6.6, python3-minimal, 3.6.5-3ubuntu1
PythonDetails: /usr/bin/python2.7, Python 2.7.15rc1, python-minimal, 2.7.15~rc1-1
RelatedPackageVersions:
 dpkg 1.19.0.5ubuntu2
 apt 1.6.3ubuntu0.1
SourcePackage: grub2
Title: package grub-pc 2.02-2ubuntu8.6 failed to install/upgrade: installed grub-pc package post-installation script subprocess returned error exit status 1
UpgradeStatus: No upgrade log present (probably fresh install)