Problem during do-release-upgrade

Trying to upgrade our VIRL server getting the following error:

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