We recently upgraded a server to Windows Server 2008, and some of our old applications decided that they no longer want to work. We are now having problems with the asp:ListView object, constantly getting the error Unknown server tag ‘asp:ListView’.

I’ve found a few articles (quite a few) that all point to the web.config file, saying that you need to add these lines:

<assemblies>
  <add assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
</assemblies>

<pages>
  <controls>
    <add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
    <add tagPrefix="asp" namespace="System.Web.UI.WebControls" assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
  </controls>
</pages>

The best (and only) solution for the problem Unknown server tag ‘asp:ListView’ on Windows Server 2008 is to completely uninstal the .NET Framework 3.5 and the .NET Framework 3.5 SP1. Download them from Microsoft – Do not install them through the Server Roles feature in Windows Server 2008 – and reinstall them from the installation packages.

Hope this helps!