Error PageRequestManagerTimeoutException

Question

I have this piece of code in Vb.NET:

Dim Script As New System.Text.StringBuilder
    If String.IsNullOrEmpty(Er1) Then
        With Script
            .Append("<script language='javascript'>")
            .Append("window.open('" & StampaUrl & "','_blank')")
            .Append("</script>")
        End With
 ScriptManager.RegisterStartupScript(btnStampa, Me.GetType, "startUp", Script.ToString, False)

where StampaUrl It is of the type: http://localhost:8281//files/***/cache/uy5ffdzagqwct0fgswv4retk/0CCB2154-0771-4BE3-820A-C4C01B2DB03B.PDF

Gives me this error in browser:

Error: Sys.WebForms.PageRequestManagerTimeoutException: The server request timed out.

How can I fix ?