Custom UserControl is not registering in ASP.NET - asp.net

Question

Update: J0e3gan tried my code in his own project, and it worked fine (with a minor correction), so the problem appears to be with Visual Studio itself, rather than the code or markup. I have tried adding a new UserControl as well, and it was not recognized either. However, VS is recognizing the AjaxControlToolkit that is registered in web.config just fine. [/update]
I'm trying to add a custom UserControl named AdminControls to the site I'm working on, but I keep getting the following error:
Element 'ControlName' is not a known element. This can occur if there is a compilation error in the Web site, or the web.config file is missing.
I'm running Visual Studio Pro 2013, and the project is a Web Application. I have searched the Web for solutions, mostly here on Stack Overflow, and have tried every one I could find, but have had no luck. There must be something I'm missing.
Here is the markup for AdminControls:
<asp:Table ID="tblAdminControls" runat="server">
<asp:TableRow>
<asp:TableCell style="min-width: 50%;"> </asp:TableCell>
<asp:TableCell style="width: 6em" id="tdCP" runat="server">
<asp:Button Font-Bold="true" Font-Size="Smaller" Font-Overline="false" ID="btnCP"
runat="server" CssClass="Button" Text="Control Panel" />
</asp:TableCell>
<asp:TableCell style="width: 5em">
<asp:Button Font-Bold="true" Font-Size="Smaller" Font-Overline="false" ID="btnLogOut"
runat="server" CssClass="Button" Text="LogOut" />
</asp:TableCell>
</asp:TableRow>
</asp:Table>
And here is the codebehind for it:
Public Class AdminControls
Inherits System.Web.UI.UserControl
Private Sub Page_Init(sender As Object, e As EventArgs) Handles Me.Init
If Request.ServerVariables("SCRIPT_NAME") = "/frmAdminCP.aspx" Then
tdCP.Visible = False
End If
End Sub
Private Sub btnCP_Click(sender As Object, e As EventArgs) Handles btnCP.Click
Response.Redirect("frmAdminCP.aspx", False)
End Sub
Private Sub btnLogOut_Click(sender As Object, e As EventArgs) Handles btnLogOut.Click
Session.RemoveAll()
Session.Abandon()
Response.Redirect("frmLogin.aspx", False)
End Sub
End Class
I've tried registering AdminControls both in the web.config file and on the page. Here's the relevant bits from the web.config file:
<pages controlRenderingCompatibilityVersion="3.5" clientIDMode="AutoID">
<controls>
<add tagPrefix="ajaxToolkit" assembly="AjaxControlToolkit" namespace="AjaxControlToolkit" />
<add tagPrefix="klc" tagName="AdminHeader" src="~/AdminControls.ascx" />
</controls>
</pages>
And last but not least, here's one of the pages I'm trying to put AdminControls in:
<%# Page Title="" Language="VB" MasterPageFile="~/MasterPage.master" AutoEventWireup="false"
EnableEventValidation="false" Inherits="ProjectName.frmAdminCP" CodeBehind="frmAdminCP.aspx.vb" %>
<%# Register TagPrefix="klc" TagName="AdminHeader" Src="~/Controls/AdminControls.ascx" %>
<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" runat="Server">
<div id="content">
<h1>Admin <span class="pagetitle">Control Panel</span> </h1>
<klc:AdminHeader runat="server" id="ahControls" />
<div>
<h2>Users</h2>
View Existing Users<br />
Add a New User
<h2>Usage Reports</h2>
View User Activity Report<br />
View Administrative Log
<h2>Project Management</h2>
View Projects<br />
Manage Project Files
</div>
</div>
</asp:Content>
I have attempted every fix I've come across, even if they seemed ridiculous. I've:
Purged the schema cache.
Cleaned and rebuilt the solution.
Restarted Visual Studio.
Restarted my computer.
Moved the file to a subdirectory.
Cut and pasted the code and markup to the same place and resaved the files.
There are no build errors, and the tagPrefix does not appear in the Intellisense auto-complete dropdown.
Can anyone see any errors I've made, or does anyone know of a solution I haven't tried yet? I haven't tried slaughtering a black rooster over my PC yet, but I'm getting close.

Answer 1

Using the code you provided, I successfully included your user control in a page - screenshot below. I was unable to reproduce the error you are getting.
The only error I encountered along the way stemmed from the two different paths you used for AdminControls.ascx:
src="~/AdminControls.ascx" your Web.config excerpt.
Src="~/Controls/AdminControls.ascx" in your (frmAdminCP.aspx) page excerpt.
Once I made the paths consistent (with each other and the scrach web app project I created), AdminControls showed up in frmAdminCP.aspx just fine:
In case it helps you, for my sanity check I simply created a new ASP.NET Web Forms Application project in Visual Studio 2013 targeting .NET 4.0, added a new Web Forms User Control item named AdminControls.ascx, added a new Web Form item named frmAdminCP.aspx, and pasted your code in all the appropriate places.

Answer 2

I understand I'm a bit late to this answer, but I found a solution.
I was experiencing the same issue where I was attempting to register a custom control but Visual Studio wasn't recognizing it. I was able to resolve this by going to Build->Build Page. After VS finished analyzing and building the page, the error/warning I was getting went away.