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: