On December 29 2011 Microsoft published security update Microsoft Security Bulletin MS11-100 - Critical. The security update has been released to fix ASP.NET DoS vulnerability. The update puts a limit to number of key-value pairs that can be sent in a single HTTP POST request. By default not more than 500 variables can be uploaded per one request.
If Image Uploader is configured to upload all files in a single HTTP POST request, it can exceed this amount which leads to failed upload requests. If it happens, error 500 (Operation is not valid due to the current state of the object) is thrown. The problem can be addressed by reconfiguring the uploader or changing your web server settings. Find the detailed information how to fix this issue in this topic: PRB: Image Uploader ASP.NET generates "Operation is not valid due to current state of object" error.