Try adding this into the system.web section of your web.config file:
pages enableEventValidation="false"
@ Page Language="C#" MasterPageFile="~/Admin/MasterPage_Admin.master" AutoEventWireup="true" CodeFile="CategoryManagement.aspx.cs" Inherits="Admin_CategoryManagement" Title="Untitled Page" EnableEventValidation="false"
You can also benefit from those features by leaving it enabled then register your control for event validation. Simply add the following call in the PreRender or Render page life cycle then your control should work without having to turn off eventValidation:Page.ClientScript.RegisterForEventValidation(this.UniqueID);