I think you have used FTB Editor in your page, Please add
ValidateRequest="false"
as below
<![CDATA[<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" MaintainScrollPositionOnPostback="true" ValidateRequest="false" %>]]>
I am not sure, may this will help you, please reply if your problem get solved or face another issue.
Thanks