This error comes when you try to get user input in raw html through a textbox control in order to resolve this you will have to include ValidateRequest="false"
in
<%@ Page Title="" ValidateRequest="false" Language="C#" MasterPageFile="" AutoEventWireup="true" CodeBehind="" Inherits="" %>
and also you will have to include this
<configuration>
<system.web>
<compilation debug="true" targetFramework="4.0" />
<httpRuntime requestValidationMode="2.0"/>
</system.web>
</configuration>
in your web.config file
73bd2903-d7a8-4d8c-9fcf-143846edcbb0|0|.0|96d5b379-7e1d-4dac-a6ba-1e50db561b04
ASP.NET
html in textbox