package flash-kernel 3.98ubuntu12 failed to install/upgrade: »installiertes flash-kernel-Skript des Paketes post-installation«-Unterprozess gab den Fehlerwert 1 zurück

error occurred during distro upgrade on a raspberry pi nextcloud box.
I think the problem consists in a firmware partition on the system, that is too small to keep all files and backup files during the update. I had to manually delete these files already several times in order to make installation possible.

ProblemType: Package
DistroRelease: Ubuntu 20.04
Package: flash-kernel 3.98ubuntu12
ProcVersionSignature: Ubuntu 4.4.0-1017.23-raspi2 4.4.13
Uname: Linux 4.4.0-1017-raspi2 armv7l
ApportVersion: 2.20.11-0ubuntu27.9
Architecture: armhf
CasperMD5CheckResult: skip
Date: Thu Oct 8 21:07:45 2020
ErrorMessage: »installiertes flash-kernel-Skript des Paketes post-installation«-Unterprozess gab den Fehlerwert 1 zurück
Python3Details: /usr/bin/python3.8, Python 3.8.5, 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 2.0.2ubuntu0.1
SourcePackage: flash-kernel
Title: package flash-kernel 3.98ubuntu12 failed to install/upgrade: »installiertes flash-kernel-Skript des Paketes post-installation«-Unterprozess gab den Fehlerwert 1 zurück
UpgradeStatus: Upgraded to focal on 2020-10-08 (0 days ago)
modified.conffile..etc.default.flash-kernel:
 LINUX_KERNEL_CMDLINE="quiet splash"
 LINUX_KERNEL_CMDLINE_DEFAULTS=""
mtime.conffile..etc.default.flash-kernel: 2020-03-26T11:53:16.575225