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
Same problem as before. After upgrading the categories would not show in the sidebar. Since there wasn't any code change with respect to this JavaScript "error:categoryTreeLoadChildren is not defined", I played with all sorts of config settings and bingo I found one that works:
# Extension: Categorytree
$wgUseAjax = true;
require_once("{$IP}/extensions/CategoryTree/CategoryTree.php");
$wgCategoryTreeSidebarRoot = "Contents";
$wgCategoryTreeForceHeaders = true;
$wgCategoryTreeSidebarOptions['mode']=CT_MODE_CATEGORIES;
$wgCategoryTreeDynamicTag = false;
$wgCategoryTreeDisableCache = false;
$wgCategoryTreeMaxDepth = array(CT_MODE_PAGES => 2, CT_MODE_ALL => 2, CT_MODE_CATEGORIES => 3);
See http://edutechwiki.unige.ch/en/
BTW this also makes the category tree work with Semantic Forms. My prior config did not !!
http://edutechwiki.unige.ch/mediawiki/index.php?title=Semantic_Form_demo_page&action=formedit