Can't do release upgrade from 18.04 to 20.04 (maybe due to missing candidates for packages)

[Andy Fischer]

No candidate ver: libperl5.22
No candidate ver: linux-image-4.13.0-32-generic
No candidate ver: linux-image-4.13.0-36-generic
No candidate ver: linux-image-4.13.0-37-generic
No candidate ver: linux-image-4.13.0-38-generic
No candidate ver: linux-image-4.13.0-39-generic
No candidate ver: linux-image-4.13.0-41-generic
No candidate ver: linux-image-4.13.0-43-generic
No candidate ver: linux-image-4.15.0-126-generic
No candidate ver: linux-image-extra-4.13.0-32-generic
No candidate ver: linux-image-extra-4.13.0-36-generic
No candidate ver: linux-image-extra-4.13.0-37-generic
No candidate ver: linux-image-extra-4.13.0-38-generic
No candidate ver: linux-image-extra-4.13.0-39-generic
No candidate ver: linux-image-extra-4.13.0-41-generic
No candidate ver: linux-image-extra-4.13.0-43-generic
No candidate ver: linux-modules-4.15.0-126-generic
No candidate ver: linux-modules-extra-4.15.0-126-generic
No candidate ver: lxc-common
No candidate ver: mariadb-client-10.0
No candidate ver: mariadb-server-10.0
No candidate ver: php7.0-curl
No candidate ver: postgresql-9.5

============================
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=18.04
DISTRIB_CODENAME=bionic
DISTRIB_DESCRIPTION="Ubuntu 18.04.5 LTS"

============================
ubuntu-release-upgrader-core:
  Installiert: 1:18.04.42
  Installationskandidat: 1:18.04.42
  Versionstabelle:
 *** 1:18.04.42 500
        500 http://de.archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
        500 http://de.archive.ubuntu.com/ubuntu bionic-updates/main i386 Packages
        100 /var/lib/dpkg/status
     1:18.04.17 500
        500 http://de.archive.ubuntu.com/ubuntu bionic/main amd64 Packages
        500 http://de.archive.ubuntu.com/ubuntu bionic/main i386 Packages

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.42
ProcVersionSignature: Ubuntu 4.15.0-136.140-generic 4.15.18
Uname: Linux 4.15.0-136-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.23
Architecture: amd64
CrashDB: ubuntu
Date: Sat Feb 27 10:32:18 2021
PackageArchitecture: all
ProcEnviron:
 TERM=screen-256color
 PATH=(custom, no user)
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2021-02-27 (0 days ago)