error when upgrading from v14.04 to v15 -- always receiving msg panel stating: "Could not calculate the upgrade". None of the 3 causes listed apply

[dan3city]

have attempted the upgrade every 2-3 days for the past two weeks. have applied other maintenance than was available. this is a simple, unmodified Ubuntu 14.04 image installed from the Ubuntu site download in Dec, 2015. Operating on a Lenovo T500 laptop.

Ubuntu 14.04.3 LTS release 14.04

I get the exact same error each time. First a warning message that "some download sources are disabled" (no info about which sources and I only use the options of the SW installer). Then upgrade proceeds until the "Unable to Calculate" panel is given. Then the upgrade process is backed-out.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubuntu-release-upgrader-core 1:0.220.8
ProcVersionSignature: Ubuntu 3.13.0-74.118-generic 3.13.11-ckt30
Uname: Linux 3.13.0-74-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.19
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Wed Jan 13 16:13:15 2016
InstallationDate: Installed on 2014-07-05 (557 days ago)
InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Release amd64 (20140417)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to trusty on 2016-01-14 (0 days ago)
VarLogDistupgradeTermlog:

mtime.conffile..etc.update.manager.release.upgrades: 2016-01-01T12:41:32.353555

[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.