*) Starting compiz via setting 'Visual Effects' to 'Normal' in 'Appearance Preferences' (and removing all other compiz/fusion-icon autostart entries) works fine. That's why I couldn't reproduce the problem with a new user - 'Normal' is default and compiz was already running.
Ok, this gconfd-2 thing seems to be the result of 'compiz --replace' starting a loop that triggers this bug: https:/ /bugs.launchpad .net/ubuntu/ +source/ metacity/ +bug/150721
Addition to above points:
*) Starting compiz via setting 'Visual Effects' to 'Normal' in 'Appearance Preferences' (and removing all other compiz/fusion-icon autostart entries) works fine. That's why I couldn't reproduce the problem with a new user - 'Normal' is default and compiz was already running.