Try removing the following line from your web.config
<add assembly="System.Web.Extensions, Version=18.104.22.168, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
Clean the solution ( right click on the solution file and choose "Clean Solution"). Check your references from the GAC again. The MSDN says that the same file exists in two places, or that you forgot to change the version number.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items