ASP/C# code behind can not reach control from markup

Question

Issue:
I have a markup like this (only the important lines):

<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="RTDeluxe.ascx.cs"
Inherits="MainSolution.CONTROLTEMPLATES.Kunde.RTDeluxe" %>
<ul id="linkUl" class="teaserLinksUL" runat="server"/>

The code-behind:

namespace MainSolution.CONTROLTEMPLATES.Kunde
public partial class RTDeluxe : UserControl
{
    protected void Page_Load(object sender, EventArgs e)
    {
        linkUl.InnerHtml = string.Empty;
    }
}

I can access the ul inside the code-behind and get no compilation error. But, when I debug the code I get a NullReferenceException because linkUl is NULL.

First I thought that the namespaces are the reason. But, after several tries, I'm sure that they're correct. The FileLocation seems to be correct and the controltemplates folder of my iis has a "Kunde" folder with the corresponding ascx files in it.

I have other .ascx files with the same structure -> they're working like a charm.

Question:
Are there any other reasons than the namespace for such behaviour? Do you have any hints where I can look at?

Edit:
The RTDeluxe.ascx.designer.cs file exists, the generated linkUl looks like this:

protected global::System.Web.UI.HtmlControls.HtmlGenericControl linkUl;

Edit2:
Ok, I will try to answer all your questions. Thanks for your time and feedback!

1. I have restarted Visual Studio -> The Problem persists.
2. I also have cleaned up the solution and deployed a new one. -> The problem persists.
3. When I debug and check the control hierachy I can see that the label is NOT there.
4. When I change the ID the compiler throws an error in the code-behind (which is right). If i change the ID there two I get the same behavoiur as before.
5. I also restarted my IIS and the whole pc -> No changes.
6. I have added a Name attribute to the linkul-definition -> No changes.
7. When I try to use FindControl it returns NULL.
8. The target-framework is .NET 3.5
9. The linkul is NOT inside a repeater or any other controls.
10. Removing/changing the web.config does also not lead to a solution.
11. Adding EnsureChildControls before accessing the linkUl doesnt change anything.
12. Moving the code into Page_PreRender does also not work.

I will try out your suggestions not listed here and add them soon.

Edit3:
Here the full markup:

<%@ Assembly Name="$SharePoint.Project.AssemblyFullName$" %>
<%@ Assembly Name="Microsoft.Web.CommandUI, Version=14.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>
<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="RTDeluxe.ascx.cs" Inherits="MainSolution.CONTROLTEMPLATES.Kunde.RTDeluxe" %>

<ul id="linkUl" class="teaserLinksUL" runat="server"/>

*Edit4:
Ok here some additional info I found out: When I change something in the markup, like adding plain html text it's NOT recognized by or shown in the browser. When i do something like this:

   Label label1 = new Label();
   label1.Text = "hugo lives!";
   Controls.Add(label1);

It is shown. It seems like in visual studio everything is fine... But "live" at the server the code-behind speaks to some weird different markup...

Answer 1

This might help You a bit:

On the code-behind file of the user-control at class level, add this code:

protected global::System.Web.UI.HtmlControls.HtmlGenericControl linkUl = new System.Web.UI.HtmlControls.HtmlGenericControl();

and remove the protected global::System.Web.UI.HtmlControls.HtmlGenericControl linkUl; from RTDeluxe.ascx.designer.cs file

It might be because its Object was just declared not created. Hope this helps..