Package manager fails to update ("failed to fetch")

Try changing server in software centre. The US server may be being worked on.

I tried that twice, plus I've had the problem for seventeen days now.

There's a proxy server, but I didn't change any of the settings between when it was working and when it started failing. As far as I know the company didn't change the server settings either, since none of my coworkers are having the problem.

Create a clean sources.list without 'lnRelease' and switch to main server.
https://help.ubuntu.com/community/Repositories/Ubuntu#Download%20Server
e.g.
http://paste.ubuntu.com/650898/

'lnRelease' doesn't exist anymore since the release is in the past.
404 error indicates that the requested resource may be available again in the future
https://secure.wikimedia.org/wikipedia/en/wiki/HTTP_404
Test.
http://archive.ubuntu.com/ubuntu/dists/natty/InRelease

Okay, I'll try this when I get back to work on Monday.

I completely replaced my sources.list without lnRelease, but it still seems to be using it:

Failed to fetch http://archive.ubuntu.com/ubuntu/dists/natty/InRelease
Failed to fetch http://archive.ubuntu.com/ubuntu/dists/natty-updates/InRelease
Failed to fetch http://extras.ubuntu.com/ubuntu/dists/natty/InRelease
Failed to fetch http://archive.ubuntu.com/ubuntu/dists/natty-security/InRelease
Some index files failed to download. They have been ignored, or old ones used instead.

And yes, I tried the main server and the US server as well.

Please pastebin sources.list and also have a look in /etc/apt/sources.list.d.

There recently were two questions of people who have problems because their router or ISP interfere in handling of "404 file not found" messages

To check if that is also the case here, can you please open your web browser and try to access
http://archive.ubuntu.com/ubuntu/dists/natty/InRelease
What do you see?

And please open a terminal and execute the following commands

lsb_release -a
sudo apt-get update
sudo apt-get upgrade

and then copy ALL output and paste it into a new message of this thread.

Sam: The directory is completely empty according to ls -lAh. I did a diff with your pastebin and my sources.list, and they're exactly the same.
Manfred - I get a 404 with the link you sent me, and this is the output:

root@dargueta-Precision-WorkStation-T3400:/etc/apt# lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 11.04
Release: 11.04
Codename: natty
root@dargueta-Precision-WorkStation-T3400:/etc/apt# apt-get update
Err http://extras.ubuntu.com natty InRelease

Err http://extras.ubuntu.com natty Release.gpg
  Unable to connect to extras.ubuntu.com:http:
0% [Connecting to archive.ubuntu.com (91.189.88.46)]

It also tried 91.189.92.169 - .171, and 91.189.88.30 - .31 for archive.ubuntu.com. (I wasn't watching it the entire time, so it might've tried a few others.) I left it there for over ten minutes and that's as far as it got, so I killed it after that.

Okay, it's fixed. Apparently one of the updates deleted /etc/apt/apt.conf which had a few company-specific settings in there.

apt.conf is used for proxy.
https://help.ubuntu.com/community/AptGet/Howto#Setting%20up%20apt-get%20to%20use%20a%20http-proxy