So today I tried to swap some desktop packages and got not very helpful:

tasksel: aptitude failed (100)

Common solution if you google it is to run:

apt-get update

and fix whatever errors, if you get one. I guess most common would be missing GPG keys for some of repositories used. Another one would be to run:

apt-get -f install

to fix whatever broken packages are.

But in my case none of these helped. I found real reason why tasksel was failing by running it in test mode:

tasksel -t install kubuntu-full

I was presented with following output, suggesting what kind of command is run in order to install this scenario:

debconf-apt-progress -- apt-get -q -y install kubuntu-full^

Next thing I tried was to run "apt-get -q -y install kubuntu-full^" manually to find out that some dependencies are broken. After working out these problems tasksel happily installed scenario I needed. You are welcome :)