Upgrading from 14.04 to 15.04 fails on calculating upgrade

[Tbsc]

When trying to upgrade Ubuntu either through the terminal (sudo do-release-upgrade) or through the software updater, it will fail just after starting saying that "An unresolvable error occurred while calculating the upgrade" and it can't continue.

The error I got is this:
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.

I've looked around for a solution, but I couldn't find any so I'm reporting it. I don't know if duelbooting Windows 10 on the same drive could potentially do something, but that's why I'm reporting this bug. Looking at the upgrade log files (/var/log/dist-upgrade/main.log), I noticed that close to the end it errors on trying to run SystemUnLock() and fails as it is already unlocked (I'm just typing what it said).

1) Release version: 14.04.3 LTS Trusty Tahr
2) Version of package: 1.0.220.8
3) What I expected to happen: The upgrade process to continue normally.
4) What happened instead: The upgrade failed and notified without much information.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubuntu-release-upgrader-core 1:0.220.8
ProcVersionSignature: Ubuntu 3.13.0-76.120-generic 3.13.11-ckt30
Uname: Linux 3.13.0-76-generic x86_64
NonfreeKernelModules: fglrx
ApportVersion: 2.14.1-0ubuntu3.19
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Fri Jan 22 07:42:06 2016
InstallationDate: Installed on 2015-01-20 (366 days ago)
InstallationMedia: Ubuntu 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.2)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to trusty on 2016-01-22 (0 days ago)
mtime.conffile..etc.update.manager.release.upgrades: 2016-01-21T21:18:22.748253

[Brian Murray (brian-murray)]

Thank you for taking the time to report this bug and helping to make Ubuntu better. This particular bug has already been reported and is a duplicate of bug 1534374, so it is being marked as such. Please look at the other bug report to see if there is any missing information that you can provide, or to see if there is a workaround for the bug. Additionally, any further discussion regarding the bug should occur in the other report. Please continue to report any other bugs you may find.