do-release-upgrade fails and no helpful info on how to continue

[Jona Sassenhagen]

16.04 -> 18.04 do-release-upgrade fails and tells me that I should consider if I am doing any of:

* Upgrading to a pre-release version of Ubuntu
* Running the current pre-release version of Ubuntu
* Unofficial software packages not provided by Ubuntu

It's not 1 or 2. But for 3, I have 100s of packages installed, should I go through all of them by hand? Is there a principled way of identifying which could be the source of the problem? I have CUDA installed, from NVIDIA's sources, but I would prefer to not wildly experiment with removing and reinstalling something as annoying to install as CUDA.

Apologies if the answer is simply "yes" - I am just thinking that there probably is an easier way of proceeding from here, which do-release-upgrade could more helpfully inform the user about.

1) The release of Ubuntu you are using, via 'lsb_release -rd' or System -> About Ubuntu
Ubuntu 16.04.5 LTS
2) The version of the package you are using, via 'apt-cache policy pkgname' or by checking in Software Center
N: Unable to locate package do-release-upgrade
N: Unable to locate package ubuntu-release-upgrader
3) What you expected to happen
Upgrade to 18.04
4) What happened instead
Above error message :(

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.25
ProcVersionSignature: Ubuntu 4.15.0-33.36~16.04.1-generic 4.15.18
Uname: Linux 4.15.0-33-generic x86_64
NonfreeKernelModules: nvidia_uvm nvidia_drm nvidia_modeset nvidia
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
CrashDB: ubuntu
Date: Tue Sep 4 15:24:40 2018
InstallationDate: Installed on 2016-08-04 (761 days ago)
InstallationMedia: Ubuntu 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160719)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2018-09-04 (0 days ago)