update-manager crashes, apt-get, ubuntu-bug and software center do not work too

For about a week the update manager crashes while initializing the package information.

I'm a bit helpless because a bug report (using ubuntu bug) could not be generated and apt-get update does also not work. I will provide any information I have and will do furhter analysis if you give me instructions.

The unity panel contains a notification icon stating that problems occured while running the update-manager. The user shall run the update-manager manually (1) or run apt-get in a terminal (2). As mentioned before there was a problem when running ubuntu-bug (3).

= 1 run the update-manager manually =

The application displays an error message and states a bug report should be placed and the following error message should be attached:
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E:Encountered a section with no Package: header, E:Problem with MergeList /var/lib/apt/lists/extras.ubuntu.com_ubuntu_dists_precise_main_binary-i386_Packages, E:Die Paketliste oder die Statusdatei konnte nicht eingelesen oder geöffnet werden (Translation: The package list or the status file could not be read or opened).
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= 2 Run apt-get in a terminal =

Running apt-get in a terminal failed. There were several http 404 and some GPG errors. You will find the apt.out in the attachment. I attached /etc/apt/sources.list too.

= 3 Problems while running ubuntu-bug (this seems to be already reported as Bug #996522) =

While trying to generate a bug report using "ubuntu-bug update-manager" ubuntu-bug failed and the following error message was displayed:
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Ungültiger Problembericht - Konnte Paket- oder Quellpaket-Namen nicht bestimmen.
(Translation: Invalid issue report - couldn't determine package- or source-name)
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= Version Information =

markus@markus-K52Jc:~$ lsb_release -rd
Description: Ubuntu 12.04 LTS
Release: 12.04

I was not able to determine the version of the update-manager used because "apt-cache policy update-manager" fails with the same error message as "apt-get upgrade" does (see attachment). I tried to use the Software Center without luck (It crashes just after start. The error report can not be generated. The error is the same displayed by ubuntu-bug.)