When I try to upgrade from Ubuntu Gnome 13.10 to Ubuntu 14.04, the upgrade application fails with the following message:

Could not calculate the upgrade. 

An unresolvable problem occurred while calculating the upgrade.`

Here is the complete log:

$ cat /etc/issue

Ubuntu 13.10 

 l

$ sudo apt-get update

$ sudo apt-get upgrade

$ sudo do-release-upgrade

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



Aborting

Reading package lists... Done    

Building dependency tree          

Reading state information... Done

Building data structures... Done 

Am I missing something? or it is just a bug?

UPDATE:
I analyzed the log file and found that do-release-upgrade is complaining about some obsolete packages.

$ cat /var/log/dist-upgrade/main.log | grep Obsolete

2014-04-28 09:16:23,678 DEBUG Obsolete: flashplugin-nonfree-extrasound:i386 (..)

After removing those packages, I was able to proceed with the upgrade.