If you install CategoryTree into another directory than $IP/extensions/CategoryTree/ (of course this different path is
correctly specified in LocalSettings.php with the usual "require once('$my_path')" call) the extension loads (according to
Special:Version) but refuses to do anything if you try to click on the + of sub categories.
If you change the path to the mentioned default name (and change as well the call in LocalSettings.php to te standard of
course ;-) everything works.