Session vars lost with ActiveX

I've got a curious problem occurring on only one of my apache/php servers:
While uploading with UI the session-vars are deleted. This occurs both with the java- and activex version, and only on one server. All other session-related things work smoothly on this server, just the upload by IU triggers the deletion of the vars. The session_id() maintains it's value. Naturally i checked the php.ini and other server-settings (especially session-related stuff), but i can't find an explanation.

This is a test.php script i use, it posts back to itself, ignoring the upload:
====================
<?php
session_start();
$_SESSION[date('Y-m-d h-m-s')] = 'test'; //adds a new session-var named 'current time' with value 'test' on each request
var_dump($_SESSION);
$site_domain = "http://www.somedomain.eu/";
?>
<br />
<script src="<?php echo($site_domain); ?>iu/Scripts/iuembed.js" type="text/javascript"></script>
<script type="text/javascript">
var iu = new ImageUploaderWriter("ImageUploader1", 650, 400);
iu.activeXControlEnabled = true;
iu.javaAppletEnabled = false;
iu.activeXControlCodeBase = "<?php echo($site_domain); ?>iu/Scripts/ImageUploader6.cab";
iu.addParam("LicenseKey", "XXXXX-XXXXX-XXXXX-XXXXX-XXXXX");
iu.addParam("Action", "");
iu.addParam("RedirectUrl","<?php echo($site_domain); ?>temp/test.php");
iu.writeHtml();
</script>
====================

Note that the expected behaviour is that every call on this script a new session-var $_SESSION['yyyy-mm-dd hh-mm-ss'] with value 'test' is added, and this works as expected with a F5 refresh on all servers, but when i upload an image with IU all session-vars are gone on only 1 specific server.

I hope someone has any clue what might be causing this session-loss? (both activex- and java version result in the session-loss on this server).

Still no solution found for this weird problem.
A bit more details: PHP 5.2.9 on Apache/2.2.10 (Linux/SUSE) Server.
The website has multiple upload-systems (i mean forms with POST and FILE upload), and the other ones work as expected.
While uploading the session is maintained. When upload is finished the line session_start() in the recieving file is called and at that moment the session-vars get cleared (while maintaining the session_id). When i remove this session_start() on the receiving file the session is maintained (but i can't use the session-vars in this script naturally).
Also there is a suhosin protection (0.9.27) installed on the server, but disabling it (by suhosin.simulation = "on") doesn't help either.
I've checked the headers too, and changed the IE-header into 'Image Uploader' so this value isn't changed on upload (might something check this against session-hijacking) but this...doesn't help too.
Because this behaviour doesn't happen on my other servers, and both ActiveX and Java have this problem, and i've spent way too much time on this problem, i think there is a glitch in the Matrix and my only option is to re-install this server...

Hello,

In your phpinfo I see that the name of session cookie is PHPSESSID. I tried to load your test page and saved request dump. I see that the only cookie passed to this request from your server is one with empty name (see the screenshot attached to this post). It seems that the problem is in session id cookie.

Edited by user Tuesday, September 1, 2009 10:13:46 PM(UTC)  | Reason: Not specified

Thanks Tamila, but i'm afraid this is not the problem.

I see in your screenshot it is the first visit (new session). On first visit the session-value is never sent in the calling header (mine named PHPSESSID, on the aurigma site it is called ASP.NET_SessionId). This is only on the second and following visits from the browser. This behaviour on my site is identical to the aurigma site.

The big question remains why my session-data is lost after the upload with IU on some sites. Ive tested some things and know that the session is maintained while uploading, but on finishing uploading (or cancelling) the session-data is lost. The session-data is maintained in all other functions on my sites (including uploading binary files).

Re-installing the server from scratch did not have a positive result. I also had the script tested on a server owned by the local university, same result: session-data lost.

Since it took me a bit too long i've programmed around this, but am still curious what setting might be causing this. Older servers (apache 2.0, PHP 5.0) don't seem to have this problem, and some newer servers don't have this problem too. Of course i've compared the php.ini and apache settings on working vs not working servers, but have not found a solution other then sending the ID not by cookie but by URL (which is less safe and should be avoided).

Thanks for the reply.
Although this is not even checked by my server, and my session-data is maintained with different user-agents in one session, i have tried this. I used a custom User-agent header in the script (so all request-headers, from both IE and IU, read 'Image Uploader'), this didn't help too... sniff...