How to resolve "a potentially dangerous request. Form field value was detected from client ct100_hiddept?

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C#

ASP.NET

I am getting "A potentially dangerous Request. Form Field value was detected from client ct100_hidDept on clicking button. My application is running in framework 3.5.

I have tried the below fixes. Still issue remains

What I have tried:

Code tried:
Page level added validaterequest
<pre><%@ Page Title="" Language="C#" MasterPageFile="~/MasterPage.master" AutoEventWireup="true" ValidateRequest="false"
    CodeFile="UserManagement.aspx.cs" Inherits="UserManagement" MaintainScrollPositionOnPostback="true" %>

In web.config file added validaterequest

<pages validateRequest="false">