Olaf,
Hope you can test it in a simple environment, not in your project.
And then try the args.Delete();
If it works for simple environment, that means there maybe something special which prevent the uploader release the temp file.
If it's a bug of AjaxUploader , we will fix it.
You can also test the UploadType property ,set it to IFrame/Silverlight/Flash and test again.
If we get more details , we can fix it more quickly.
Regards,
Terry