Recently I got this error message, basically telling me I was using an unknown server control. In this case, the ListView control. First thing that comes to mind is if the control itself exists in .NET 3.5, which I was using. Of course it does!
Next thing, I was reading about the solution to this being to completely reinstall .NET Framework 3.5 and .NET Framework 3.5 SP1.
However, I solved it by adding two lines to the Web.Config controls element instead, saving me a lot of time.
Here's the code:
<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>