system update from 18.04 to 20.04 fails, unresolvable error when setting up package sources

[Gernot]

output of lsb_release -rd:
Description: Ubuntu 18.04.5 LTS
Release: 18.04

output of uname -r:
4.15.0-118-generic

system update from 18.04 to 20.04 fails,
after reading new package sources an unresolvable error occurs, package sources are rolled back to prior state,
custom ppas have been disabled manually before the system update,
system stays on version 18.04

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.38
ProcVersionSignature: Ubuntu 4.15.0-118.119-generic 4.15.18
Uname: Linux 4.15.0-118-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.17
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: LXDE
Date: Fri Oct 2 17:50:39 2020
InstallationDate: Installed on 2019-02-02 (607 days ago)
InstallationMedia: Lubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-10-02 (0 days ago)
VarLogDistupgradeTermlog:

[Brian Murray (brian-murray)]

Could you please readd the sane git PPA, install the packages which it provides and try the upgrade again? Thanks in advance and good luck.

[Launchpad Janitor (janitor)]

[Expired for ubuntu-release-upgrader (Ubuntu) because there has been no activity for 60 days.]