Recently I was working on AdRotator controll in Asp.Net 2.0, at one stage while adding Ads.xml file.
I notice following error:
Error:
The AdRotator adRtrCtrl/adRotator1 could not find the AdvertisementFile or the file is invalid.
Possible causes:
- Check XML file if it is in well format. 
 - Check <NavigateUrl> tag, if it is of the form http://www.csharpcorner.com/adv=Y&ID=123 .
The ampresand & is a reserved charactor in XML so you have to use &. 
 - If the nature of your XML file is something of the form as listed below.
 
<Advertisements>
  <Adv>
    <ImageUrl>Images/1.gif</ImageUrl>
    <NavigateUrl>http://www.c-sharpcorner.com</NavigateUrl>
    <Alt>Csharp code</Alt>
    <Caption>Csharp Site</Caption>
  </Adv>
</Advertisements>
If the first element in above file is other than <Ad> then also it gives error, it has to have very first child element as <Ad>, this is what I have observed with the code.
So it takes form like.
<Advertisements>
  <Ad>
    <ImageUrl>Images/1.gif</ImageUrl>
    <NavigateUrl>http://www.c-sharpcorner.com</NavigateUrl>
    <Alt>Csharp code</Alt>
    <Caption>Csharp Site</Caption>
  </Ad>
</Advertisements>
Code for Default.aspx:
<%@ Page Language="C#" AutoEventWireup="true"  CodeFile="Default.aspx.cs" Inherits="_Default" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
 
<html xmlns="http://www.w3.org/1999/xhtml" >
<head>
    <title>Untitled Page</title>
</head>
<body>
    <form id="form1" runat="server">
    <div style="text-align: left">
         <table>
            <tr>
                <td style="width: 259px">
                    <asp:Label ID="Label1" runat="server" Font-Names="Verdana" Font-Size="Smaller" Text="AdRotator 
                      Component randomly picks up ad banners from XML file and clicking on  banner takes to the specified 
                      URL" Width="301px"></asp:Label></td>
                <td style="width: 496px">
                    <asp:AdRotator ID="adRtrCtrl" Target="_self" runat="Server" />
                </td>
            </tr>
        </table>
    </div>
    </form>
</body>
</html>
Code for Ads.xml file:
<Advertisements>
  <Ad>
    <ImageUrl>Images/1.gif</ImageUrl>
    <NavigateUrl>http://www.c-sharpcorner.com</NavigateUrl>
    <Alt>Csharp code</Alt>
    <Caption>Csharp Site</Caption>
  </Ad>
  <Ad>
    <ImageUrl>Images/1.gif</ImageUrl>
    <NavigateUrl>http://www.vbdotnetheaven.com</NavigateUrl>
    <Alt>VB.Net</Alt>
    <Caption>Visual Basic</Caption>
  </Ad>
  <Ad>
    <ImageUrl>Images/3.gif</ImageUrl>
    <NavigateUrl>http://www.c-sharpcorner.com</NavigateUrl>
    <Alt>Csharp code</Alt>
    <Caption>Csharp Site</Caption>
  </Ad>
  <Ad>
    <ImageUrl>Images/4.gif</ImageUrl>
    <NavigateUrl>http://www.mindcracker.com</NavigateUrl>
    <Alt>.Net Professional Job Site</Alt>
    <Caption>VS.Net</Caption>
  </Ad>
</Advertisements>
Note: I have tested the code 5-6 time with above error.