Upgrades from 13.04 to 13.10 fail

[Dave Perry]

I have not had any issued with past upgrades (e.g. from 12.10 to 13.04).
but on two different machines after clicking 'upgrade' after a normal
Software Updater is complete, I get the following error

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.

On both machines I had removed the proprietary graphics drivers and was
running with nouveau.  I also had deselected any non ubuntu ppa's that I
had added.  So the failure is unlikely due to Unofficial software packages.
The other two listed possible causes don't apply.

Is this a known issue?

Thanks for any suggestions.
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