Is it a default page that's missing? What's the URL? Are you clicking on a link that's generating this message? What's the URL? What's generating the URL? To be honest, this is almost certainly a web programming issue rather than a SQL Server issue!
@Chama Please elaborate on what do you perceive as uploading. Does it mean that you have an **input type="file"** on the html form and then you try to save the file to the web server? Do you use any custom/third party controls to do that or just the native .NET control? Error 404 hints that there is a code which tries to redirect the response to the page which does not (yet, any longer) exist, and there could be a number of reasons why it is happening. For example, if you have a code setting the value of the URL to redirect to but some error takes place and is handled BEFORE that value is set then the URL will be invalid, and you will get error 404. In other words, please describe your problem, maybe post some code snippet.