Comment 2 for bug 1557096

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