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Well, some sources of softwares aren't working; mostly all the universe and multiverse sources' applications are not installable... shortly the problem has shown in the picture in the link below. Please help!

enter image description here

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possible duplicate of How do I fix a "Failed to download package files" error? – Jorge Castro Jul 2 '12 at 21:57
Scroll to the far right, we can't see the full error in that message. Are you getting 404s? – Thomas W. Jul 23 '12 at 18:31

If possible, could you edit your question to show the two lines in full that we can see part of in your screenshot? I don't think this in an internet connection-related problem. If it was, ALL of the repositories would have failed to download if this was the case, not just one or two. There are a couple of possibilities that I am aware of that could cause this.

  1. As mentioned in Tareq's answer, perhaps the Ubuntu server for your area is temporarily having problems. If this is the case, there should be mention of a 404 error at the end of each line that failed to download. Things like this should be fixed in a matter of only a day or two in most cases.

  2. Try running an update from the terminal and see if you get the same (or different) errors. Simply open a terminal and run the following command:

    sudo apt-get update

Wait for it to finish and then see if you still have errors.

  1. Perhaps for some reason you are missing one or more GPG Keys. These keys are a security function that helps to keep the repositories safe. If this is the case, you should see something like NO_PUBKEY or something similar to this screenshot at the end of each line:

enter image description here

The best way to make sure you have all of your correct GPG Keys is to install launchpad-getkeys from the WebUpd8 repository. Do that by entering the following lines in the terminal:

sudo add-apt-repository ppa:nilarimogard/webupd8
sudo apt-get update
sudo apt-get install launchpad-getkeys

After that, simply run the command:

sudo launchpad-getkeys

You will see something like this: enter image description here

As it says, you shouldn't see anymore missing key errors. When the program finishes, simply run Update Manager again or use the above commands to update from the Terminal.

sudo apt-get update

again and see if you still have the same problem.

Once again, I do NOT believe this is an internet issue unless it is on the other end. Only one or two of your repositories failed to download, not ALL of them, and you can still use the internet, so if this is an internet-related issue it is on the end of the repository servers, as far as I know. Hope this helps!

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their errors are 404s – Thomas W. Jul 23 '12 at 18:31

That's an internet problem! Are you able to access internet? if yes , and this still happens, then may be you're behind proxy with authentication. You'll then need to provide proxy with password.

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I think if it was strictly an internet problem, more than a couple of the repositories would be broken. – Shawn Nov 27 '11 at 13:28
404s are the issue, i think, not issues with the internet. – Thomas W. Jul 23 '12 at 18:31

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