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You should not upgrade from 14.04 to 15.04, this is probably a bug in the update manager since you can not skip releases.
– xanguaJan 23 '16 at 17:13
1
If it is really a bug, it should be closed here and you should write a bug report on launchpad instead. However if there's a workaround or quick fix one can manually apply, then that should be posted as answer and the question should stay open.
– Byte CommanderJan 25 '16 at 19:45