do-release upgrade fails from trusty to xenial

[Jon]

When running do-release-upgrade:

Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade.

This can be caused by:
* Upgrading to a pre-release version of Ubuntu
* Running the current pre-release version of Ubuntu
* Unofficial software packages not provided by Ubuntu

$ cat /var/log/dist-upgrade/main.log | grep ERROR
2018-01-05 14:51:21,346 ERROR Dist-upgrade failed: 'E:Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.'

$ grep Broken /var/log/dist-upgrade/apt.log | wc
    617 9846 72781

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubuntu-release-upgrader-core 1:0.220.9
ProcVersionSignature: Ubuntu 4.4.0-104.127~14.04.1-generic 4.4.98
Uname: Linux 4.4.0-104-generic x86_64
NonfreeKernelModules: nvidia_uvm nvidia_drm nvidia_modeset nvidia
ApportVersion: 2.14.1-0ubuntu3.27
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Fri Jan 5 14:46:47 2018
InstallationDate: Installed on 2017-01-05 (365 days ago)
InstallationMedia: It
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to trusty on 2018-01-05 (0 days ago)