Could be that the upload times out. Try to increase the execution time out in web.config.

<httpRuntime maxRequestLength="1000000" /> in web.config system.web section

Also have a look at this link and add <requestLimits> in web.config

The section would like below ( under <system.webServer> section )

      <security>
           <requestFiltering>
              <requestLimits maxAllowedContentLength="52428800">            
              </requestLimits>
         </requestFiltering>
      </security>

Thanks for all your replies.

I have tried them all but it wont work for the user in IE7 or Firefox. When I'm using the same useraccount it works fine.

According to bandwidth tests, the users network is a lot faster than mine so I doubt that this is a timeout issue.

Also bear in mind that ASP.net wont timeout when running in debug mode. So when you try to reproduce yourself ensure you are compiled to release you have debug=false. This post explains it well:

http://aspalliance.com/1341_The_Infamous_DebugTrue_Attribute_in_ASPNET.all

Thanks but all my testing has been made on the same production server where the user has had the problems.