“Not found” error after idle time

I am working on a Silverlight WCF RIA Services application. If the user lets the application wait a while, for instance 10 minutes, while he is looking at something else and returns to the application, then the next data request will fail with a “Not found” error.

I tried all sorts of tricks to get the application to return a proper error, since it seems that “Not Found” is the stand error returned, hiding the real error. None of the things I found helped. It turns out that it is a simple ASP timeout, that can be fixed by adding the following timeout to web.config:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <system.web>
    <authentication mode="Forms">
      <forms name=".myApp_ASPXAUTH" timeout="180" />
    </authentication>
    <!-- Other configurations -->
  </system.web>
</configuration>

This gives a timeout of 3 hours (180 minutes).

Another solution would be to have the client poll the server at regular intervals to prevent a timeout.