Error in ASP.Net: Sys.WebForms.PageRequestManagerParserErrorException

RaisKazi: Thanks for your post. Checked but it is not the case. As mentioned in my inital post, I transplaned the exact the same code from another aspx. file (in the same project). Another one works fine. Somethin is stranger to me.
I re-checked my code in both aspx (1 works well but another 1 not). The code for the one working well is below:

<asp:ImageButton ID="ImageButton1" runat="server" ImageUrl="~/Images/PDF.jpg"
    onclick="ImageButton1_Click" style="height: 16px"/>

and the code behind:

protected void ImageButton1_Click(object sender, ImageClickEventArgs e) {
    WebClient client = new WebClient();
    Byte[] buffer = client.DownloadData(path);
    Response.ContentType = "application/pdf";
    Response.AddHeader("content-length", buffer.Length.ToString());
    Response.BinaryWrite(buffer);
}

The one does not working likes that below:

<div>
    <asp:ScriptManager ID="ScriptManager1" runat="server"/>

    <asp:updatePanel ID="UpdatePanel1" runat="server">
        <ContentTemplate>
            <asp:PlaceHolder runat="server" ID="placeholder1"/>
        </ContentTemplate>
    </asp:updatePanel>
</div>

The code behind runs the same code in the method:

public void imageBtn_Click(object sender, ImageClickEventArgs e)   {
    ......
    string pdfPath = g.sFolder + myPdf;
    WebClient client = new WebClient();
    Byte[] buffer = client.DownloadData(pdfPath);
    Response.ContentType = "application/pdf";
    Response.AddHeader("content-length", buffer.Length.ToString());
    Response.BinaryWrite(buffer);
}

How can this problem be solved? Thanks if you can provide your hint.

The problem has been solved by referring the post at .[^]. I added
ScriptManager scriptManager = ScriptManager.GetCurrent(this.Page);
...
scriptManager.RegisterPostBackControl(imageBtn);
in the Page_Load().