Ubuntu release upgrader from Ubuntu 15.10 to Ubuntu 16.04 stops without doing the upgrade.

[Guillermo Burastero]

Soon after beginning the upgrade process from Ubuntu 15.10 to Ubuntu 16.04 it stops without correctly finished it.
A splash message windows opened with the following text in Spanish:

==

No se ha podido calcular la actualización

Eso se ha podido causar por:
* Actualizar a una versión de Ubuntu aún no publicada
* Estar ejecutando la versión actual, aún no publicada, de Ubuntu
* Paquetes no oficiales de software no proporcionados por Ubuntu

Si nada de esto aplica, informe de este error usando la orden «ubuntu-bug ubuntu-release-upgrader-core» en una terminal.

==

Then only possible reason may be the third: "Not official software packets provided by Ubuntu", but I can not imaging which.

Thanks in advance.

Guillermo.

<email address hidden>
Bahía Blanca, Argentina
+(54-9291) 574-3173 or [phone]

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: ubuntu-release-upgrader-core 1:15.10.14.4
ProcVersionSignature: Ubuntu 3.16.0-29.39-generic 3.16.7-ckt2
Uname: Linux 3.16.0-29-generic x86_64
ApportVersion: 2.19.1-0ubuntu5
Architecture: amd64
CrashDB: ubuntu
Date: Mon Apr 25 11:05:17 2016
InstallationDate: Installed on 2014-10-14 (559 days ago)
InstallationMedia: Ubuntu 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.2)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to wily on 2016-04-25 (0 days ago)
VarLogDistupgradeTermlog: