12.04 -> 14.04 dist upgrade fails

[palz]

do-release-upgrade fails to upgrade the system from 12.04 to 14.04.

it fails on "calculating changes" stage, outputs "Could not calculate the upgrade"

sources.list.d/ is empty

in apt.log there are a lot of Broken reports like

Broken apport:i386 Depends on python3 [ i386 ] < 3.2.3-0ubuntu1.2 -> 3.4.0-0ubuntu2 > ( python )

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.17
ProcVersionSignature: Ubuntu 3.2.0-68.102-generic-pae 3.2.62
Uname: Linux 3.2.0-68-generic-pae i686
ApportVersion: 2.0.1-0ubuntu17.7
Architecture: i386
Date: Wed Aug 27 00:49:26 2014
GsettingsChanges:

InstallationMedia: Xubuntu 11.04 "Natty Narwhal" - Release i386 (20110427)
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 LANG=C
 SHELL=/bin/bash
SourcePackage: update-manager
Symptom: ubuntu-release-upgrader-core
UpgradeStatus: Upgraded to precise on 2014-08-26 (0 days ago)
VarLogDistupgradeTermlog: