It looks the package ubuntu-minimal wasn't found on the mirror you are using. I think this was a temporary issue and you should try the upgrade again.
>From Mainlog.txt: 2016-03-15 02:05:54,147 DEBUG need_server_mode(): run in 'desktop' mode, (because of pkg 'lubuntu-core') 2016-03-15 02:05:54,148 ERROR No 'ubuntu-minimal' available/downloadable after sources.list rewrite+update 2016-03-15 02:05:54,149 ERROR 'ubuntu-minimal' was not downloadable 2016-03-15 02:06:11,622 DEBUG abort called ** Changed in: ubuntu-release-upgrader (Ubuntu) Status: New => Incomplete -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1557096 Title: cannot upgrade to 16.04 To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/1557096/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs