do-release-upgrade fails with just 'An unresolvable problem occurred while calculating the upgrade'

[trozen]

do-release-upgrade fails with a message:

```
An unresolvable problem occurred while calculating the upgrade.

If none of this applies, then please report this bug using the
command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal. If
you want to investigate this yourself the log files in
'/var/log/dist-upgrade' will contain details about the upgrade.
Specifically, look at 'main.log' and 'apt.log'.
```

However, there's not really significant information in the logs.

ProblemType: Bug
DistroRelease: Ubuntu 23.04
Package: ubuntu-release-upgrader-core 1:23.04.7
ProcVersionSignature: Ubuntu 6.2.0-37.38-generic 6.2.16
Uname: Linux 6.2.0-37-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.26.1-0ubuntu2.1
Architecture: amd64
CasperMD5CheckResult: unknown
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Wed Nov 29 09:33:55 2023
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to lunar on 2023-11-29 (0 days ago)