[Bug 1888837] [NEW] failed when upgrading from 1804 to 2004

[X.A]

Public bug reported:

```
Checking package manager
Reading package lists... Done    
Building dependency tree          
Reading state information... Done

Calculating the changes

Calculating the changes

Could not determine the upgrade

An unresolvable problem occurred while calculating the upgrade.

This was likely caused by: 
* Unofficial software packages not provided by Ubuntu 
Please use the tool 'ppa-purge' from the ppa-purge 
package to remove software from a Launchpad PPA and 
try the upgrade again. 

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'. 

```

No furthur info was given on which package caused the issue.
I've investigate for some time and cannot figure out why.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.34 [origin: unknown]
ProcVersionSignature: Ubuntu 5.4.0-42.46~18.04.1-generic 5.4.44
Uname: Linux 5.4.0-42-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.9-0ubuntu7.14
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Fri Jul 24 13:41:37 2020
InstallationDate: Installed on 2019-08-31 (327 days ago)
InstallationMedia: Ubuntu 18.04.3 LTS "Bionic Beaver" - Release amd64 (20190805)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-07-24 (0 days ago)

** Affects: ubuntu-release-upgrader (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: amd64 apport-bug bionic dist-upgrade third-party-packages

-- 
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Ubuntu.
https://bugs.launchpad.net/bugs/1888837

Title:
  failed when upgrading from 1804 to 2004

Status in ubuntu-release-upgrader package in Ubuntu:
  New

Bug description:
  ```
  Checking package manager
  Reading package lists... Done    
  Building dependency tree          
  Reading state information... Done

  Calculating the changes

  Calculating the changes

  Could not determine the upgrade

  An unresolvable problem occurred while calculating the upgrade.

  This was likely caused by: 
  * Unofficial software packages not provided by Ubuntu 
  Please use the tool 'ppa-purge' from the ppa-purge 
  package to remove software from a Launchpad PPA and 
  try the upgrade again. 

  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'. 

  ```

  No furthur info was given on which package caused the issue.
  I've investigate for some time and cannot figure out why.

  ProblemType: Bug
  DistroRelease: Ubuntu 18.04
  Package: ubuntu-release-upgrader-core 1:18.04.34 [origin: unknown]
  ProcVersionSignature: Ubuntu 5.4.0-42.46~18.04.1-generic 5.4.44
  Uname: Linux 5.4.0-42-generic x86_64
  NonfreeKernelModules: nvidia_modeset nvidia
  ApportVersion: 2.20.9-0ubuntu7.14
  Architecture: amd64
  CrashDB: ubuntu
  CurrentDesktop: ubuntu:GNOME
  Date: Fri Jul 24 13:41:37 2020
  InstallationDate: Installed on 2019-08-31 (327 days ago)
  InstallationMedia: Ubuntu 18.04.3 LTS "Bionic Beaver" - Release amd64 (20190805)
  PackageArchitecture: all
  SourcePackage: ubuntu-release-upgrader
  UpgradeStatus: Upgraded to bionic on 2020-07-24 (0 days ago)

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