Markus,
Can you post some code.
I suggest you test it step by step.
You can create a new empty website and test the AJAX at first (without Sharepoint)
The probably reason is that there's exception while posting the form.
so you can try this script , it shall alert before the error.
<script>
function CuteWebUI_AjaxUploader_OnPostback()
{
alert("ready to submit the form , guidlist : " + this.value );
}
</script>
Regards,
Terry