I got "An unresolvable problem occurred while calculating the upgrade." while trying to upgrade to focal fossa. No further information about the problem was given, just the command for reporting the problem.

[Rainer Schulze-Pillot]

I got "An unresolvable problem occurred while calculating the upgrade." while trying to upgrade from 18.04 to 20.04. No further information about the problem was given before the "If none of this applies", just after that and an empty line the command for reporting the problem. Before trying to upgrade I installed all available updates to 18.04, and then got the message that my system was up to date but the upgrade was available.

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
Date: Tue Sep 29 13:16:18 2020
InstallationDate: Installed on 2018-08-14 (776 days ago)
InstallationMedia: Ubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-09-29 (0 days ago)
VarLogDistupgradeTermlog:

[Rainer Schulze-Pillot (rschupi)]

Hm, strange, but after I shut down my computer and started it new and then started the software updater, it proceeded happily and without further complaint with "updating" (not "upgrading").
It's running now, I hope the system will be fine in the end.

[Rainer Schulze-Pillot (rschupi)]

Everything went through smoothly now. Maybe this was connected to the fact that my first attempt to upgrade stopped with an "unresolvable problem" with the advice to disable ppas. I did that using synaptic, reloaded repository information and started the next try to upgrade, which was then stopped as described in my original report above.
Anyway, all problems are gone now, for whatever reason.