upgrade to 11.10 failed

It failed and asked to fill a bug report. Something about dependencies.

At first, it said "No valid mirror found" and asked to rewrite sources.list - I agreed. Then it said "Third party sources disabled". And then:

Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade:
E:Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.

 This can be caused by:
 * Upgrading to a pre-release version of Ubuntu
 * Running the current pre-release version of Ubuntu
 * Unofficial software packages not provided by Ubuntu

If none of this applies, then please report this bug using the command 'ubuntu-bug update-manager' in a terminal.

I am not sure if point 3 applies or not, but two above surely don't apply.

One more thing - after update is cancelled, sources list is full with ancient entries about lucid and maverick.

ProblemType: Bug
DistroRelease: Ubuntu 11.04
Package: update-manager 1:0.150.3
ProcVersionSignature: Ubuntu 2.6.38-11.48-generic 2.6.38.8
Uname: Linux 2.6.38-11-generic i686
NonfreeKernelModules: nvidia
Architecture: i386
CheckboxSubmission: b346f016d264f3ef62407d5e29c198a2
CheckboxSystem: daed2f3d6643b4a84b4520a2427f8c2b
Date: Fri Sep 16 12:18:36 2011
GConfNonDefault:
 /apps/update-manager/check_new_release_ignore=
 /apps/update-manager/first_run=false
 /apps/update-manager/show_details=true
 /apps/update-manager/window_size=(1024,695)
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=en_US:en
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: update-manager
UpgradeStatus: Upgraded to natty on 2011-09-16 (0 days ago)