UnicodeDecodeError on do-release-upgrade to 20.04

[Thomas E]

Running Kubuntu 19.10, also on multiseat with LightDM
Package: ubuntu-release-upgrader-qt, version is 1:19.10.15.4. I had to submit the package as ubuntu-release-upgrader, because apparently the QT version isn't in vanilla Ubuntu's repositories.

I'm unable to upgrade to 20.04. The graphical upgrader (run from Krunner) crashes after clicking "Upgrade".

Here is the output of "do-release-upgrade":

Checking for a new Ubuntu release
Get:1 Upgrade tool signature [1,554 B]
Get:2 Upgrade tool [1,342 kB]
Fetched 1,343 kB in 0s (0 B/s)
authenticate 'focal.tar.gz' against 'focal.tar.gz.gpg'
extracting 'focal.tar.gz'

Reading cache

Traceback (most recent call last):
  File "/tmp/ubuntu-release-upgrader-si9p302g/focal", line 8, in <module>
    sys.exit(main())
  File "/tmp/ubuntu-release-upgrader-si9p302g/DistUpgrade/DistUpgradeMain.py", line 235, in main
    save_system_state(logdir)
  File "/tmp/ubuntu-release-upgrader-si9p302g/DistUpgrade/DistUpgradeMain.py", line 128, in save_system_state
    scrub_sources=True)
  File "/tmp/ubuntu-release-upgrader-si9p302g/DistUpgrade/apt_clone.py", line 149, in save_state
    self._write_state_installed_pkgs(sourcedir, tar)
  File "/tmp/ubuntu-release-upgrader-si9p302g/DistUpgrade/apt_clone.py", line 194, in _write_state_installed_pkgs
    pkg.name, pkg.installed.version, int(pkg.is_auto_installed))
  File "/usr/lib/python3/dist-packages/apt/package.py", line 599, in version
    return self._cand.ver_str
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xf5 in position 14: invalid start byte
Error in sys.excepthook:
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/problem_report.py", line 477, in add_to_existing
    self.write(f)
  File "/usr/lib/python3/dist-packages/problem_report.py", line 430, in write
    block = f.read(1048576)
  File "/usr/lib/python3.7/codecs.py", line 322, in decode
    (result, consumed) = self._buffer_decode(data, self.errors, final)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0x8b in position 1: invalid start byte

Original exception was:
Traceback (most recent call last):
  File "/tmp/ubuntu-release-upgrader-si9p302g/focal", line 8, in <module>
    sys.exit(main())
  File "/tmp/ubuntu-release-upgrader-si9p302g/DistUpgrade/DistUpgradeMain.py", line 235, in main
    save_system_state(logdir)
  File "/tmp/ubuntu-release-upgrader-si9p302g/DistUpgrade/DistUpgradeMain.py", line 128, in save_system_state
    scrub_sources=True)
  File "/tmp/ubuntu-release-upgrader-si9p302g/DistUpgrade/apt_clone.py", line 149, in save_state
    self._write_state_installed_pkgs(sourcedir, tar)
  File "/tmp/ubuntu-release-upgrader-si9p302g/DistUpgrade/apt_clone.py", line 194, in _write_state_installed_pkgs
    pkg.name, pkg.installed.version, int(pkg.is_auto_installed))
  File "/usr/lib/python3/dist-packages/apt/package.py", line 599, in version
    return self._cand.ver_str
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xf5 in position 14: invalid start byte

ProblemType: Bug
DistroRelease: Ubuntu 19.10
Package: ubuntu-release-upgrader-core 1:19.10.15.4
ProcVersionSignature: Ubuntu 5.3.0-51.44-generic 5.3.18
Uname: Linux 5.3.0-51-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu8.8
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: KDE
Date: Tue May 19 12:59:35 2020
InstallationDate: Installed on 2019-12-19 (152 days ago)
InstallationMedia: Kubuntu 19.10 "Eoan Ermine" - Release amd64 (20191017)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
Symptom: ubuntu-release-upgrader
UpgradeStatus: Upgraded to eoan on 2020-05-19 (0 days ago)