Hi,

There are multiple problems can cause this error:

1. Try to create a blank new empty project and test AJAXUploader there. This will tell you whether it has to do with other elements in your project. AJAXUploader will not work if you have other modules that try to fetch the form data in a very early stage. This module may have been introduced not in your application's web.config, but somewhere else, such as a web.config file in the parent folder;

2. Try to increase httpRuntime -> executionTimeout value. When this value is too small and your file is too big, ASP.NET will terminate the request;

3. If you run IIS 7, check your request filtering settings. You can find more information here:

http://doc.essentialobjects.com/library/1/ajaxuploader/troubleshoot.aspx

The easiest way to verify whether IIS 7 request filtering is the problem is to try uploading a big file with a regular HTML file input element (or standard ASP.NET file FileUpload control). If that fails, then it may have to do with IIS 7 request filtering.

Hope this helps. Please let us know if that resolves the issue.

Thanks!