release upgrade (saucy->trusty): "unresolvable problem calculating updates", "could not determine which system updates are available"

[Klaus]

Upgrade failed at step 2 (registering new package sources) with error msg mentioned in summary.

$ lsb_release -rd
Description: Ubuntu 13.10
Release: 13.10

$ apt-cache policy ubuntu-release-upgrader-core
ubuntu-release-upgrader-core:
  Installiert: 1:0.205.6
  Installationskandidat: 1:0.220.2
  Versionstabelle:
     1:0.220.2 0
        500 http://de.archive.ubuntu.com/ubuntu/ trusty/main amd64 Packages
 *** 1:0.205.6 0
        100 /var/lib/dpkg/status

ProblemType: Bug
DistroRelease: Ubuntu 13.10
Package: ubuntu-release-upgrader-core 1:0.205.6
ProcVersionSignature: Ubuntu 3.11.0-20.35-generic 3.11.10.6
Uname: Linux 3.11.0-20-generic x86_64
ApportVersion: 2.12.5-0ubuntu2.2
Architecture: amd64
CrashDB: ubuntu
Date: Tue May 13 21:46:35 2014
InstallationDate: Installed on 2012-07-19 (662 days ago)
InstallationMedia: Ubuntu 11.10 "Oneiric Ocelot" - Release amd64 (20111012)
MarkForUpload: True
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to saucy on 2014-05-13 (0 days ago)
VarLogDistupgradeTermlog: