Actually the problem might be a different one. I realized that none of the following applications works properly:
1) mintupdate: starts and then nothing happens (tried both from the menu and from terminal)
2) synaptic: starts only from terminal with
sudo synaptic
3) user management from system settings; it works only from terminal with
sudo kcmshell4 userconfig
but then lots of error messages relating to
/tmp/kde-user is owned by uid 1000 instead of uid 0
; using the command
kdesudo kcmshell4 userconfig
results instead in the following error:
kdesudo kcmshell4 userconfig
QInotifyFileSystemWatcherEngine::addPaths: inotify_add_watch failed: No such file or directory
QFileSystemWatcher: failed to add paths: /home/claudius/.config/ibus/bus
No protocol specified
kcmshell4: cannot connect to X server localhost:0.0
Totally confused!