stops updating with error unknown, just suggestions.

[Robert S M Wulff]

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

If none of this applies, then please report this bug using the command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.26
ProcVersionSignature: Ubuntu 4.15.0-43.46~16.04.1-generic 4.15.18
Uname: Linux 4.15.0-43-generic x86_64
NonfreeKernelModules: nvidia_uvm nvidia_drm nvidia_modeset nvidia
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
CrashDB: ubuntu
Date: Thu Dec 20 18:32:28 2018
InstallationDate: Installed on 2017-03-14 (646 days ago)
InstallationMedia: Ubuntu 16.04.2 LTS "Xenial Xerus" - Release amd64 (20170215.2)
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=en_US
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2018-12-21 (0 days ago)
VarLogDistupgradeTermlog: