Since 2 weeks I always had problems installing software on my beloved Ubuntu Linux.
There was always some 404 Page Not Found Error on some Server, even with "apt-get update".
Honestly, I didn't have the time to go after that, because the semester is really rolling these days. And I'm not a Linux genius, you know. I had no iea where to start looking.

Today, after googling a little, I remembered editing the file /etc/apt/sources.list right after installing Ubuntu. I inserted some backport then, as it is (still!) proposed on the Ubuntu Forum . You specify that some "backport"-Server will be
"http://ubuntu-backports.mirrormax.net/":
In detail, you ought to write this:
"deb http://ubuntu-backports.mirrormax.net/ hoary-backports main universe multiverse restricted"

However, on the Ubuntu backport project page it says:

"NOTE: This hoary-backports line is obsolete. Please don't use it:
deb http://ubuntu-backports.mirrormax.net/ ..."
Now I thought I found it. I just edited /etc/apt/sources.list, replacing the URL with
one of the other mirrors that are proposed there.
They all failed, too.

I love remembering those seconds where you are so close to giving up. I browsed to one of those mirror servers to look if there is anything at all. I already had noticed that "http://ubuntu-backports.mirrormax.net/" contains no files for the distribution "hoary" which happens to be mine.
But "http://acm.cs.umn.edu/ubp/" has everything there. It's just in a directory with a slightly different label. Instead of "dists/hoary-backports/" it is "dists/hoary-backports-staging/". Seeing that, I tried using that in /etc/apt/sources.list:

"deb http://acm.cs.umn.edu/ubp/ hoary-backports-staging main universe multiverse restricted"

and voila - it worked. "apt-get update" finally yielded no errors and Synaptic now installed what I was looking for.

With so much documentation that Ubuntu has build up, it is no big surprise that it is not easy to be updated at all places (though I think this backport server list is a little flawed - you need to know the directory structure on the server...).