I deployed an ASP.NET  application to another server and the page that includes a few ReportViewer controls started showing the "ASP.NET session has expired" error in each report. The same web app worked totally fine on my box. I didn’t have time to investigate before I moved on to other work and then a few days later I came across a solution to the problem by accident when reading a KB article related to RS security on WebHost4Life.com. They suggested changing the AsyncRendering property to false. Since there are some performance penalties when turning off asynchronous rendering, I decided to make it a setting in Web.config and turn it off only if needed:

      bool asyncRendering = bool.Parse(ConfigurationManager.AppSettings.Get("AsyncRendering"));
      rptvAverageOrder.ServerReport.ReportServerUrl = rsUrl;

After I tried that, the error went away but I ended up with another issue. Reports with images (in my case charts) ended up having a missing image with that ugly placeholder with an X in it. I did some googling and found out that this can happen if asynchronous rendering in ReportViewer is off and the server name contains an underscore as it does in my case so these two issues are related. When I use the IP address or "localhost" in the URL, everything works fine regardless of how this property is set. Also, some people pointed out is that if you turn off asynchronous rendering, reports get messed up when viewed with Firefox. I will report this as an issue on Microsoft Connect and I guess in the meantime we have to avoid deploying ReportViewer applications on servers with underscores in their names.