package zfsutils-linux 0.8.3-1ubuntu12.10 failed to install/upgrade: installed zfsutils-linux package post-installation script subprocess returned error exit status 1

[Erik Elfgren]

When I try to install or upgrade any package, the process gets stuck saying:
root@Home:# dpkg --configure -a
Setting up zfsutils-linux (0.8.3-1ubuntu12.10) ...
zfs-import-scan.service is a disabled or a static unit, not starting it.
zfs-import-scan.service is a disabled or a static unit, not starting it.
Job for zfs-mount.service failed because the control process exited with error code.
See "systemctl status zfs-mount.service" and "journalctl -xe" for details.
invoke-rc.d: initscript zfs-mount, action "start" failed.
● zfs-mount.service - Mount ZFS filesystems
     Loaded: loaded (/lib/systemd/system/zfs-mount.service; enabled; vendor preset: enabled)
     Active: failed (Result: exit-code) since Mon 2021-08-09 18:31:32 CEST; 4ms ago
       Docs: man:zfs(8)
    Process: 3507018 ExecStart=/sbin/zfs mount -a (code=exited, status=1/FAILURE)
   Main PID: 3507018 (code=exited, status=1/FAILURE)

aug 09 18:31:32 Home systemd[1]: Starting Mount ZFS filesystems...
aug 09 18:31:32 Home zfs[3507018]: cannot mount '/': directory is not empty
aug 09 18:31:32 Home systemd[1]: zfs-mount.service: Main process exited, code=exited, status=1/FAILURE
aug 09 18:31:32 Home systemd[1]: zfs-mount.service: Failed with result 'exit-code'.
aug 09 18:31:32 Home systemd[1]: Failed to start Mount ZFS filesystems.
dpkg: error processing package zfsutils-linux (--configure):
 installed zfsutils-linux package post-installation script subprocess returned error exit status 1
dpkg: dependency problems prevent configuration of zfs-initramfs:
 zfs-initramfs depends on zfsutils-linux (>= 0.8.3-1ubuntu12.10); however:
  Package zfsutils-linux is not configured yet.

dpkg: error processing package zfs-initramfs (--configure):
 dependency problems - leaving unconfigured
Setting up linux-image-5.8.0-63-generic (5.8.0-63.71~20.04.1) ...
dpkg: dependency problems prevent configuration of zfs-zed:
 zfs-zed depends on zfsutils-linux (>= 0.8.3-1ubuntu12.10); however:
  Package zfsutils-linux is not configured yet.

dpkg: error processing package zfs-zed (--configure):
 dependency problems - leaving unconfigured
Processing triggers for initramfs-tools (0.136ubuntu6.6) ...
update-initramfs: Generating /boot/initrd.img-5.8.0-63-generic
I: The initramfs will attempt to resume from /dev/sde5
I: (UUID=ea598dd5-d1c8-4b5c-8301-867862c80a42)
I: Set the RESUME variable to override this.
Processing triggers for linux-image-5.8.0-63-generic (5.8.0-63.71~20.04.1) ...
/etc/kernel/postinst.d/dkms:
 * dkms: running auto installation service for kernel 5.8.0-63-generic
   ...done.
/etc/kernel/postinst.d/initramfs-tools:
update-initramfs: Generating /boot/initrd.img-5.8.0-63-generic
I: The initramfs will attempt to resume from /dev/sde5
I: (UUID=ea598dd5-d1c8-4b5c-8301-867862c80a42)
I: Set the RESUME variable to override this.
/etc/kernel/postinst.d/zz-update-grub:
Sourcing file `/etc/default/grub'
Sourcing file `/etc/default/grub.d/init-select.cfg'
Generating grub configuration file ...
grub-probe: error: unknown filesystem.

ProblemType: Package
DistroRelease: Ubuntu 20.04
Package: zfsutils-linux 0.8.3-1ubuntu12.10
ProcVersionSignature: Ubuntu 5.4.0-40.44-generic 5.4.44
Uname: Linux 5.4.0-40-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu27.18
AptOrdering:
 linux-headers-5.8.0-55-generic:amd64: Remove
 linux-hwe-5.8-headers-5.8.0-55:amd64: Remove
 NULL: ConfigurePending
Architecture: amd64
CasperMD5CheckResult: skip
Date: Tue Aug 10 06:36:43 2021
DpkgHistoryLog:
 Start-Date: 2021-08-10 06:36:36
 Commandline: /usr/bin/unattended-upgrade
 Remove: linux-hwe-5.8-headers-5.8.0-55:amd64 (5.8.0-55.62~20.04.1), linux-headers-5.8.0-55-generic:amd64 (5.8.0-55.62~20.04.1)
ErrorMessage: installed zfsutils-linux package post-installation script subprocess returned error exit status 1
InstallationDate: Installed on 2020-07-08 (397 days ago)
InstallationMedia: Ubuntu 20.04 LTS "Focal Fossa" - Release amd64 (20200423)
Python3Details: /usr/bin/python3.8, Python 3.8.10, python3-minimal, 3.8.2-0ubuntu2
PythonDetails: /usr/bin/python2.7, Python 2.7.18, python-is-python2, 2.7.17-4
RelatedPackageVersions:
 dpkg 1.19.7ubuntu3
 apt 2.0.6
SourcePackage: zfs-linux
Title: package zfsutils-linux 0.8.3-1ubuntu12.10 failed to install/upgrade: installed zfsutils-linux package post-installation script subprocess returned error exit status 1
UpgradeStatus: No upgrade log present (probably fresh install)
mtime.conffile..etc.default.zfs: 2020-08-05T10:29:12.101838