Comment 4 for bug 1786876

when I try upgrade to Ubuntu 18.04 from 16.04 the upgrade software stop and show the message "There is a unsolve problem, an error occurred while calculating the update.". After three possible cause are showed:
Upgrade to a pre-release
to run from the pre-release version
Use non-official ubuntu package.

Neither above are applicable to me. How to fix??

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.25
ProcVersionSignature: Ubuntu 4.15.0-30.32~16.04.1-generic 4.15.18
Uname: Linux 4.15.0-30-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: KDE
Date: Mon Aug 13 22:30:11 2018
InstallationDate: Installed on 2017-04-01 (499 days ago)
InstallationMedia: Kubuntu 16.04.2 LTS "Xenial Xerus" - Release amd64 (20170215)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2018-08-14 (0 days ago)
VarLogDistupgradeTermlog: