Someone asked how to use asp:Silverlight control in Silverlight forum. He said that he is getting the error above after adding asp:Silverlight control to ASP.NET project where he host Silverlight content.
Remove this <system.web><compilation debug=”false”><assemblies>
<add assembly=”System.Web.Extensions, Version=126.96.36.199, Culture=neutral, PublicKeyToken=31BF3856AD364E35″/>
It’s because you have added two different version of System.Web.Extension in your project. The one that you have is 188.8.131.52 but when you place asp:Silverlight control in ASP.NET, the new version (184.108.40.206) will be added in web.config. So, you have to remove this lin.
Happy Silverlighting! :)