Working solution fo MediaWiki 1.20 !
Sorry, I didn't follow this thread, therefore I didn't post a diff. Anyhow, today (6 month later) I tried again
# Extension: Categorytree
$wgUseAjax = true;
$wgCategoryTreeSidebarRoot = "Contents";
$wgCategoryTreeForceHeaders = true;
$wgCategoryTreeDynamicTag = false;
$wgCategoryTreeDisableCache = false;
$wgCategoryTreeMaxDepth = array(CT_MODE_PAGES => 2, CT_MODE_ALL => 2, CT_MODE_CATEGORIES => 3);
BTW this also makes the category tree work with Semantic Forms. My prior config did not !!