Invalid character in the given encoding. Error

Is there a reason why you've got the data file URL in the DataFile attribute twice?

The XML in the URL you're targeting doesn't play nice with .NET... you'll see what I mean in my example below.  Regardless, the main problem is that the source of this data is sticking "&" characters in the <description> element of the <item> sets... which the .NET XML parser doesn't like. I'm not sure if that's only .NET either.

Anyway, if you want to use these feeds you're going to have to figure out a way to replace the
<description></description>
tags with
<description><![CDATA[original text]]></description>
to allow illegal characters to pass through.

You may want to consider looking for some type of .NET RSS component that specifically handles situations like this if you don't want to tackle the necessary coding yourself...

Here's my example code; I got it to populate the gridview after I had massaged the incoming XML data appropriately.

This code is written targeting the NET v2.0 framework, with some tried and true v1.1 techniques for dealing with XML. Also, I wrote this in code-behind; you may need to make a few slight adjustments if you are doing this inline.
------------------------------------------------------------

'need these to get at a few necessary objects
Imports System.IO
Imports System.Xml


    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

            'This "If..Then" condition allows me to use this same functionality with more than one data source

        If Request.QueryString.Keys.Count > 0 AndAlso Request.QueryString.AllKeys.Contains("feed") Then

            'This label helps me to see that the page actually refreshed.
            Me.lblStatus.Text = "It's ALIVE!
" & Date.Now.ToString

            'Once formatted, bind the data to the control
            Me.GridView1.DataSource = GetRSSFeed(Request.QueryString.Item("feed"))
            Me.GridView1.DataBind()

        Else

            'This label helps me to see that the page actually refreshed.
            Me.lblStatus.Text = "* Please provide a valid feed URL *
" & Date.Now.ToString

        End If
    End Sub


    Function GetRSSFeed(ByVal strURL As String) As DataTable
        Dim sb As New StringBuilder

        'Get the XML data

        '@@@ Ideally, this is what we want to use...
        '          but the following line DOES NOT WORK because of "&" symbols in data
        'Using reader As XmlTextReader = New XmlTextReader(strURL)

        '@@@ We have to use this one instead to at least prove we can get at the data...
        Using reader As XmlTextReader = New XmlTextReader(Server.MapPath(".") & "\App_Data\xml_file_4330.xml")
            While reader.Read()
                Select Case reader.NodeType
                    Case XmlNodeType.Element
                        sb.AppendLine("<" & reader.Name & ">")
                    Case XmlNodeType.Text
                        sb.Append(reader.Value)
                    Case XmlNodeType.CDATA
                        sb.Append("<![CDATA[" & reader.Value & "]]>")
                    Case XmlNodeType.ProcessingInstruction
                        sb.AppendLine("<?" & reader.Name & " " & reader.Value & "?>")
                    Case XmlNodeType.Comment
                        sb.AppendLine("<!--" & reader.Value & "-->")
                    Case XmlNodeType.XmlDeclaration
                        sb.AppendLine("<?xml version='1.0'?>")
                    Case XmlNodeType.Document
                    Case XmlNodeType.DocumentType
                        sb.AppendLine("<!DOCTYPE " & reader.Name & " [" & reader.Value & "]")
                    Case XmlNodeType.EntityReference
                        sb.AppendLine(reader.Name)
                    Case XmlNodeType.EndElement
                        sb.AppendLine("</" & reader.Name & ">")
                End Select
            End While
        End Using

        'return a new DataSet
        Dim ds As DataSet = New DataSet()
        ds.ReadXml(New StringReader(sb.ToString))
        Return ds.Tables(2)
    End Function

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FINAL COMMENTS:
If you have have any influence over how your target XML files are generated, as them to use CDATA on any fields that MIGHT contain characters that XML considers illegal, such as "&".

HTH.