That error raise when some risky charecter found in Http Request object. You can ignore that by skiping validation but it is not recommended. You can skip validation check by setting ValidatinonRequestProperty to false and EnableViewStateMac to false in page directives in markup file.example
<![CDATA[<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="About.aspx.cs" Inherits="WebApp2010.About" ValidateRequest="false" EnableViewStateMac="false" %>]]>