The message was:
APT system reports E: transaction set check failed.
The question is simple:
If I want to reinstall, am I supposed to do something other* than install form the livecd and start installing new apps from syanptic?
(*: I mean: change sources.lst, aptupgrade or sth. like that)
After installation one should always open Synaptic, click Reload --> Mark All Upgrades --> Apply
again from the confirmation window to be fully upgraded
and in sync with the repos. Only then should one try to install additional
applications. Each time new application installations are desired, the same process needs to be first applied to resync the already installed packages to the current state of the repos.
Even the latest released ISO images will be out of date almost immediately. The process of updating the repo packages never stops.