Perhaps not 100% answering the question, but i stumbled onto this thread in the hope of solving a session problem when ajax-posting a fileupload from the assetmanager of the innovastudio editor. Eventually the solution was simple: they have a flash-uploader. Disabling that (setting
var flashUpload = false;
in asset.php) and the lights started blinking again.
As these problems can be very hard to debug i found that putting something like the following in the upload handler will set you (well, me in this case) on the right track:
$sn=session_name();
error_log("session_name: $sn ");
if(isset($_GET[$sn])) error_log("session as GET param");
if(isset($_POST[$sn])) error_log("session as POST param");
if(isset($_COOKIE[$sn])) error_log("session as Cookie");
if(isset($PHPSESSID)) error_log("session as Global");
A dive into the log and I quickly spotted the missing session, where no cookie was sent.