[ubuntu] can't do-release-upgrade from 14.04.5 LTS ot 16.04?

Hi,

I'm trying to do-release-upgrade from my 14.04.5 LTS box to 16.04 LTS, but do-release-upgrade tells me that either I have unofficial packages or I'm trying to upgrade from an outdated version, neither of which seems to be true. But other than this message, I can't get out the specifics of why the upgrade is not working.

14.04.5 is listed among the 'current' releases: http://releases.ubuntu.com/ . this is what my system sais:

# lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 14.04.5 LTS
Release:        14.04
Codename:       trusty

What could be wrong?

Akos