your log errors:
DEBUG failed to SystemUnLock() (E:Not locked)
2015-12-25 08:25:53,093 DEBUG need_server_mode(): run in 'desktop' mode,
(because of pkg 'ubuntustudio-desktop')
Please copy/paste the output of:
cat /etc/apt/sources.list
** Changed in: ubuntu-release-upgrader (Ubuntu)
Status: New => Incomplete
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https://bugs.launchpad.net/bugs/1529247
Title:
cannot upgrade to 15.04
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