I receive the ajax error ONLY when the following conditions are met:1. I'm using Internet Explorer.2. I try to upload a file from our network (not my local machine), and the file is large enough to invoke the progress handler.* Everything works fine when I upload the exact same file from my local machine.* And also everything works fine in Firefox, when uploading a large file from our network.Please note that the file does upload successfully, just the rad progress handler fails.Source url returned invalid content:---------------------------Windows Internet Explorer---------------------------RadUpload Ajax callback error. Source url returned invalid content: var rawProgressData = {InProgress:true,ProgressCounters:true,CurrentOperationText:'\\mynetwork\users\nprather\My Documents\Temp\big.txt',PrimaryTotal:'21.22MB',PrimaryValue:'9.48MB',PrimaryPercent:'45',SecondaryValue:'0',Speed:'9.19MB/s',TimeElapsed:'1031',TimeEstimated:'1278'};Telerik.RadUploadProgressHandler.ashx?RadUrid=c64c41ef-28b9-4829-948b-6d8290bcb45cDid you register the RadUploadProgressHandler in web.config?Please, see the help for more details: RadUpload for ASP.NET Ajax - Configuration - RadUploadProgressHandler.---------------------------OK ---------------------------