do-release-upgrade fails on 12.04LTS to 14.04LTS server error: An unresolvable problem occurred while calculating the upgrade [duplicate]

Question

do-release-upgrade fails with this error:
Checking package manager
Reading package lists... Done
Building dependency tree
Reading state information... Done
Building data structures... Done

Calculating the changes

Calculating the changes

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.


Restoring original system state

Answer 1

Try this:

First: You need to remove all ppa

Second: You need to upgrade your system.

Reboot your system to finish installing updates, and to upgrade to new available version:

Open a terminal,

Press Ctrl+Alt+T

Run it:

sudo -i
update-manager -d

Software Updater will show up and search for the new release, after a few seconds, you will see a screen like below that saying:

However, Ubuntu 14.04 is available now.

Press the button Upgrade to start upgrading to Ubuntu 14.04.

The Software Updater will ask you to confirm still you want to upgrade, press Start Upgrade to begin installing Ubuntu 14.04.

Now, the Software Updater will prepare to start setting up new software channels, and after a few minutes, the software updater will notify you the details the number of packages are going to be removed, and number of packages are going to be installed, press Start upgrade to continue.

Ended the dist-upgrade, reboot your system and:

Open a terminal,

Press Ctrl+Alt+T

Run it:

sudo -i
dpkg --configure -a
apt-get -f install
apt-get update
apt-get dist-upgrade
apt-get autoremove
apt-get clean