During upgrade from 18 to 20 it spit this error.
ProblemType: Package
DistroRelease: Ubuntu 20.04
Package: python2-minimal 2.7.17-2ubuntu4
ProcVersionSignature: Ubuntu 4.15.0-101.102-generic 4.15.18
Uname: Linux 4.15.0-101-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.14
Architecture: amd64
Date: Tue May 26 09:49:11 2020
ErrorMessage: installed python2-minimal package post-installation script subprocess returned error exit status 1
InstallationDate: Installed on 2015-03-15 (1899 days ago)
InstallationMedia: Ubuntu-Server 12.04.4 LTS "Precise Pangolin" - Release amd64 (20140204)
Python3Details: /usr/bin/python3.6, Python 3.6.9, python3-minimal, 3.6.7-1~18.04
PythonDetails: /usr/bin/python2.7, Python 2.7.18rc1, python-is-python2, 2.7.17-4
RelatedPackageVersions:
dpkg 1.19.0.5ubuntu2.3
apt 1.6.12ubuntu0.1
SourcePackage: python-defaults
Title: package python2-minimal 2.7.17-2ubuntu4 failed to install/upgrade: installed python2-minimal package post-installation script subprocess returned error exit status 1
UpgradeStatus: Upgraded to focal on 2020-05-26 (0 days ago)
It seems problem was in custom installed python-2.7 in /usr/local/bin.
Not really "vanilla" ubuntu and not ubuntu fault, but maybe should be checked properly (if that possible).