Comment 6 for bug 1354825

I experienced the same problem and, in my case, it seemed to be provoked by a bogus /var/lib/update-manager/meta-release-lts file. It contained what seemed to me an error report page generated by a server (the file looked like a html file with something like "page not available, contact the webmaster").
I think that, at some stage, ubuntu has tried to grab from the web the information that should be placed in this file and the connection failed by any reason, only garbage was retrieved and garbage was written to the file.
After deleting the file, "do-release-upgrade" worked as expected (and wrote a correct meta-release-lts file). Anyway, I aborted for the moment the upgrade (I will perform it when I have time to face any possible problem: these upgrades always rise some issue to be addressed :-) ).

Good luck.