Error 500 with KCFinder in CKEditor
My mistake, I left a hack in civicrm.settings.php (and forgot it, it was months ago) which led to this weird behavior. Reverting the hack solves the problem :

// can't remember why I did this 18 months ago, but this is obviously a bad idea...
//require_once './sites/all/modules/civicrm/CRM/Core/ClassLoader.php';
require_once 'CRM/Core/ClassLoader.php';


Many thanks to XDebug, and lesson learned (again :) about hacking CiviCRM code...