Removing xubuntu-desktop worked for me as well. Thank you all.
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system upgrade 9.10 --> 10.04 could not calculate upgrade when both
xubuntu-desktop and ubuntu-desktop are installed
https://bugs.launchpad.net/bugs/571743
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Bugs
** Attachment added: "apt.log"
http://launchpadlibrarian.net/47809127/apt.log
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could not calculate the upgrade from 9.10 to 10.04 LTS
https://bugs.launchpad.net/bugs/573228
You received this bug notification because you are a member of Ubuntu
Bugs, which is subscribed to Ubuntu.
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ubuntu
** Attachment added: "main.log"
http://launchpadlibrarian.net/47038794/main.log
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could not calculate the upgrade from 9.10 to 10.04 LTS
https://bugs.launchpad.net/bugs/573228
You received this bug notification because you are a member of Ubuntu
Bugs, which is subscribed to Ubuntu.
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ubun
Public bug reported:
Binary package hint: update-manager
A unresolvable problem occurred while calculating the upgrade. The
package 'Ubuntu-desktop' is marked for removal but it is in the removal
blacklist.
** Affects: update-manager (Ubuntu)
Importance: Undecided
Status: New
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