Wrong URL / path used for repository

[drunken monkey]

Since 8 days the update manager (and command line apt-get) seems to use a non-existent URL for one of the repositories. The error message I get is this:

W:Failed to fetch http://de.archive.ubuntu.com/ubuntu/dists/precise/universe/source/Sources 404 Not Found [IP: 141.30.13.20 80]
, E:Some index files failed to download. They have been ignored, or old ones used instead.

Checking the URL in the browser, I see there's only Sources.gz and Sources.bz2.
Does anyone know why apt would think that's the right path, or how I could change that?

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.4
ProcVersionSignature: Ubuntu 3.2.0-24.38-generic 3.2.16
Uname: Linux 3.2.0-24-generic x86_64
NonfreeKernelModules: fglrx
ApportVersion: 2.0.1-0ubuntu7
Architecture: amd64
Date: Tue May 22 17:47:49 2012
GsettingsChanges:
 com.ubuntu.update-manager first-run false
 com.ubuntu.update-manager launch-time 1337699848
 com.ubuntu.update-manager window-height 500
 com.ubuntu.update-manager window-width 600
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release amd64 (20120425)
PackageArchitecture: all
ProcEnviron:
 TERM=xterm
 PATH=(custom, user)
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: update-manager
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Thomas Kregelin (tkr)]

In the Software Settings Preferences GUI i switched the Download Source ("Download from:")

from "Server for Germany" to "Main Server".

That workaround fixed it for me.