How do I find out what is causing Edge Developer Tools to be BLANK on the company ASP.NET WebForms website?

We have an old ASP.NET WebForms site that currently only runs in Microsoft Edge because of the way hundreds of links open in popup windows. It needs to be rewritten, but that is our long-term project that has not been approved resources yet.

I recently learned how to use a timer to update Page 1 when dialog on Page 2 calls a 3rd dialog on Page 3. Page 3 sets a Session variable, and Page 1 listens for it.

This is my Page 1:

<form id="Form1" method="post" runat="server">
    <script type="text/javascript">
        document.addEventListener("DOMContentLoaded", function () {
            setTimeout(function () {
                var csrWhere = '<%=Session("csrWhere")%>'
                if (csrWhere != '') {
                    alert(csrWhere);
                    location.reload();
                }
            }, 200);
        });
    </script>
    <asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
    <telerik:RadMenu ID="mnuMain" EnableEmbeddedSkins="false" CssClass="RadMenu_Claim" EnableShadows="true" OnClientItemClicking="MenuOpenWindow" CollapseAnimation-Type="None" DataSourceID="SiteMapDataSource1" Style="z-index: 999; position: absolute; top: 3px; left: 12px" runat="server" />
    <asp:SiteMapDataSource ID="SiteMapDataSource1" ShowStartingNode="False" SiteMapProvider="XmlSiteMapProviderHome" runat="server" />

It is not firing, so I tried to pull up the Developer Tools and query some information using the Javascript console. That was when I noticed that Developer Tools will open, but the content of the page is blank:

By contrast, I can come here on the same PC running the same browser, and Developer Tools works like it should.

Obviously, something on the site is interfering with Developer Tools. We have 2 developers at our company, and neither of us have ever seen this or know what is causing it.

This website project has numerous older third party DLLs like Telerik and an older Javascript library, and I suspect it is one of those.

Does anyone know how I would find out what is blocking my Developer Tools?

Update
In the Web.config file, I found this:

<authorization>
    <allow roles="VistaUser" />
    <deny users="*" />
</authorization>

Is it possible that the Visual Studio Debugger role needs access? I'd like to test that theory, but I do not know what that role is called at this moment. I'm going to see if I can find that.

Obviously, something on the site is interfering with Developer Tools.

Actually not. Have you noticed the little IE icon in the left of your address bar:

It means you are running this page in Internet Explorer mode. The Chromium DevTools apparently won't work in IE mode.

To debug the content of an IE mode tab, use IEChooser to open Internet Explorer DevTools, as follows:

1. In Windows, open the Run dialog. For example, press Windows logo key + R.

2. Enter %systemroot%\system32\f12\IEChooser.exe, and then click OK.

3. In IEChooser, select the entry for the IE mode tab.

Of course, you may wonder why you're in IE mode. Normally, it is configured by your company (using group policies and such) for legacy application compatibility. In case you don't want to run this application in IE mode, you can contact your administrator for more information.