Do you have xubuntu-desktop installed on this machine?
There is a known bug, bug #57143 that make it problematic to update
ubuntu-desktop if xubuntu-desktop is there.
I doubt this is your problem... but it seems worth to ask.
** Changed in: update-manager (Ubuntu)
Status: New => Incomplete
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Can not mark 'ubuntu-desktop' for upgrade
https://bugs.launchpad.net/bugs/576953
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