When I click on Apply to install updates I get this message.

The package that is being modified was not found on your system or in any software origin

What can I do to solve this problem?

I've got the same thing, and so far google isn't being very helpful. I can only find links to source code and this forum. Does anybody know what's wrong?

are your ppa's consistent with the version of K being run?

is this error specific to a particular package?

update:
- looked for relevant bug, didn't see one (though might have missed it) https://bugs.kde.org/buglist.cgi?quicksearch=apper

for me, the error was caused by
flashplugin-downloader - Adobe Flash Player plugin downloader

my /etc/apt/sources.list has these ppas active, I think there's nothing unusual there:
deb http://de.archive.ubuntu.com/ubuntu/ oneiric main restricted
deb-src http://de.archive.ubuntu.com/ubuntu/ oneiric main restricted
deb http://de.archive.ubuntu.com/ubuntu/ oneiric-updates main restricted
deb-src http://de.archive.ubuntu.com/ubuntu/ oneiric-updates main restricted
deb http://de.archive.ubuntu.com/ubuntu/ oneiric universe
deb-src http://de.archive.ubuntu.com/ubuntu/ oneiric universe
deb http://de.archive.ubuntu.com/ubuntu/ oneiric-updates universe
deb-src http://de.archive.ubuntu.com/ubuntu/ oneiric-updates universe
deb http://de.archive.ubuntu.com/ubuntu/ oneiric multiverse
deb-src http://de.archive.ubuntu.com/ubuntu/ oneiric multiverse
deb http://de.archive.ubuntu.com/ubuntu/ oneiric-updates multiverse
deb-src http://de.archive.ubuntu.com/ubuntu/ oneiric-updates multiverse
deb http://security.ubuntu.com/ubuntu/ oneiric-security main restricted
deb-src http://security.ubuntu.com/ubuntu/ oneiric-security main restricted
deb http://security.ubuntu.com/ubuntu/ oneiric-security universe
deb-src http://security.ubuntu.com/ubuntu/ oneiric-security universe
deb http://security.ubuntu.com/ubuntu/ oneiric-security multiverse
deb-src http://security.ubuntu.com/ubuntu/ oneiric-security multiverse
deb http://archive.canonical.com/ubuntu/ oneiric partner
deb http://extras.ubuntu.com/ubuntu/ oneiric main
deb-src http://extras.ubuntu.com/ubuntu/ oneiric main

I think you should try K forums, as it appears to be K issue

You could remove the offending package using apt, re-run the update and then dl and install the flash-plugin deb http://packages.ubuntu.com/oneiric/i386 ... r/download - but as I don't run K I have now idea if this will actually work or if will cause issues with reloading Flash

time for a silly question, what do you mean by "K"? I assumed you meant KDE, but that would be this forum...

sorry meant Kubuntu, it might be a distro specific issue as both reporters in the thread use it

I end up installing updates with Muon software center. I have 3 programs in Kubuntu to install software: Muon, Apper and software management - KDE control module .

So I guess that the provisional solution is to use Muon software center.

I see the same thing on 11.10, on several systems. If I use "apt-get upgrade" on the command line, though, the very packages it's complaining about install just fine. This seems to me like a sign the problem isn't in the repositories or software sources, but in Apper.

Also:

I end up installing updates with Muon software center. I have 3 programs in Kubuntu to install software: Muon, Apper and software management - KDE control module .

Yes, I've noticed this too. IMHO, it looks fairly unprofessional, to have several programs all purporting to be my "software manager", but which don't really work quite right (see above, or the internal errors the GUI update tools give me from time to time). It seems the only really reliable way to update my system is to use the command line, which I've never seen fail.

This sort of thing is why, as much as I love Linux, I find it hard to recommend it to my non-technical friends and family. There are too many things that just don't work right out of the box from GUIs, like syncing to network time servers, or having to manually edit config files to prevent long delays in hostname resolution on WINS networks. It's like all the underpinnings are there, but nobody bothered to make them work right, so the end user still has to know how to poke at the underpinnings him or herself.