package tinyproxy 1.8.4-5 failed to install/upgrade: installed tinyproxy package post-installation script subprocess returned error exit status 1

Configuration file /etc/tinyproxy/tinyproxy.conf contained:
User nobody
Group nogroup

But previous installation used:
User tinyproxy
Group tinyproxy

This has probably changed after upgrade from 16.04 to 18.04.
However, existing files (and their parent folders):
/var/run/tinyproxy/tinyproxy.pid
/var/log/tinyproxy/tinyproxy.log

used uid/gid owners:
tinyproxy:adm

=> tinyproxy service failed to start because of permissions.

ProblemType: Package
DistroRelease: Ubuntu 18.04
Package: tinyproxy 1.8.4-5
ProcVersionSignature: Ubuntu 4.15.0-34.37-generic 4.15.18
Uname: Linux 4.15.0-34-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.9-0ubuntu7.4
AptOrdering:
 libreoffice-style-galaxy:amd64: Remove
 libxmlsec1-nss:amd64: Remove
 libxmlsec1:amd64: Remove
 tinyproxy:amd64: Install
 NULL: ConfigurePending
Architecture: amd64
Date: Sun Oct 7 10:26:34 2018
ErrorMessage: installed tinyproxy package post-installation script subprocess returned error exit status 1
InstallationDate: Installed on 2017-12-22 (288 days ago)
InstallationMedia: Ubuntu 16.04.3 LTS "Xenial Xerus" - Release amd64 (20170801)
PackageArchitecture: all
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: tinyproxy
Title: package tinyproxy 1.8.4-5 failed to install/upgrade: installed tinyproxy package post-installation script subprocess returned error exit status 1
UpgradeStatus: Upgraded to bionic on 2018-09-09 (27 days ago)
mtime.conffile..etc.tinyproxy.tinyproxy.conf: 2018-10-07T10:28:40.059814