When I leave the textbox txtFirstName empty and click the button only the text "Enter a name" from RequiredFieldValidator is displayed.So the ValidatorCalloutExtender does not have any effect on the displayed error message.
I use the AjaxControlToolkit for .NET 4.5 and I use VS 2013 Ultimate
In this some kind of bug it's a very basic page.
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="WebForm1.aspx.cs" Inherits="Modul_3.WebForm1" %>
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server"></asp:ToolkitScriptManager>
<asp:Label ID="Label1" runat="server" Text="FirstName" />
<asp:TextBox ID="txtFirstName" runat="server" />
<asp:RequiredFieldValidator ID="rfvFirstName" runat="server" ControlToValidate="txtFirstName" ErrorMessage="Enter a name" Display="Dynamic">
</asp:RequiredFieldValidator>
<asp:ValidatorCalloutExtender ID="ValidatorCalloutExtender2" runat="server" Width="350px"
TargetControlID="rfvFirstName">
</asp:ValidatorCalloutExtender>
<asp:Button ID="Button1" runat="server" Text="Button" OnClick="Button1_Click" />
</form>
</body>
</html>
//Tony