Upgrade cannot be computed on upgrade from 18.04 to 20.04

The attempt to upgrade generates cycles of broken dependencies, even though the initial 18.04 system has no broken packages, is upgraded, and is working correctly. The upgrade logs did not give me a clue; maybe it is something simple?

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.38
ProcVersionSignature: Ubuntu 4.15.0-112.113-generic 4.15.18
Uname: Linux 4.15.0-112-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.16
Architecture: amd64
CrashDB: ubuntu
Date: Wed Aug 19 08:41:11 2020
PackageArchitecture: all
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-08-19 (0 days ago)