This seems to have fixed the issue, the Resp.End.

Code:
                    Dim fi As New IO.FileInfo(FileName)
                    'Resp is a HttpResponse from DwnLD.aspx.vb
                    Resp.Clear()
                    Resp.ClearHeaders()
                    Resp.ClearContent()
                    Resp.AppendHeader("Content-Disposition", "attachment; filename=" & IO.Path.GetFileName(FileName))
                    Resp.AppendHeader("Content-Length", fi.Length.ToString())
                    'hmmm....
                    '                   "application/octet-stream"
                    'for .docx files
                    '                   "application/vnd.openxmlformats-officedocument.wordprocessingml.document"
                    ' Resp.ContentType = "application/vnd.openxmlformats-officedocument.wordprocessing*?ml.document"
                    Resp.ContentType = System.Web.MimeMapping.GetMimeMapping(FileName)
                    Resp.TransmitFile(FileName)
                    Resp.Flush()
                    Resp.End() '<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<