Data at the root level is invalid

<?xml version="1.0" encoding="utf-8"?>
<W-TIBCPTRs>
  <W-TIBCPTR>
    <TYPTRT>FDR2 R</TYPTRT>
    <CLAFCNO VALIDE="NON">5b1</CLAFCNO>
    <NUMCLI>0067781</NUMCLI>
    <TYPACT>D</TYPACT>
    <NATCTRT>STD</NATCTRT>
    <CLDEVCDE>EUR</CLDEVCDE>
    <AFCACH>511</AFCACH>
    <NUMACH>3700167</NUMACH>
    <DEVACH>EUR</DEVACH>
    <NUMDOC>20130051                           </NUMDOC>
    <LIBECR>D�l�gation signature(s)                                     </LIBECR>
    <GAPROFIL>JFL       </GAPROFIL>
    <GADATE>20130521</GADATE>
    <GAHEURE>113122</GAHEURE>
    <FILLER />
  </W-TIBCPTR>
  <W-TIBCPTR>
    <TYPTRT>FDR2 R</TYPTRT>
    <CLAFCNO>511</CLAFCNO>
    <NUMCLI>0068078</NUMCLI>
    <TYPACT>D</TYPACT>
    <NATCTRT>STD</NATCTRT>
    <CLDEVCDE>EUR</CLDEVCDE>
    <AFCACH>511</AFCACH>
    <NUMACH>2717150</NUMACH>
    <DEVACH>EUR</DEVACH>
    <NUMDOC />
    <LIBECR />
    <GAPROFIL />
    <GADATE />
    <GAHEURE />
    <FILLER />
  </W-TIBCPTR>
  <W-TIBCPTR>
    <TYPTRT>FDR2 V</TYPTRT>
    <CLAFCNO>511</CLAFCNO>
    <NUMCLI>0048606</NUMCLI>
    <TYPACT>D</TYPACT>
    <NATCTRT>STD</NATCTRT>
    <CLDEVCDE>EUR</CLDEVCDE>
    <AFCACH>511</AFCACH>
    <NUMACH>3129386</NUMACH>
    <DEVACH>EUR</DEVACH>
    <NUMDOC>fa2651                             </NUMDOC>
    <LIBECR>Non abouti/Facture � pr�-relancer                           </LIBECR>
    <GAPROFIL>JFL       </GAPROFIL>
    <GADATE>20130523</GADATE>
    <GAHEURE>142823</GAHEURE>
    <FILLER />
  </W-TIBCPTR>
  <W-TIBCPTR>
    <TYPTRT>FDR2 R</TYPTRT>
    <CLAFCNO>511</CLAFCNO>
    <NUMCLI>0069023</NUMCLI>
    <TYPACT>D</TYPACT>
    <NATCTRT>STD</NATCTRT>
    <CLDEVCDE>EUR</CLDEVCDE>
    <AFCACH>511</AFCACH>
    <NUMACH>3768193</NUMACH>
    <DEVACH>EUR</DEVACH>
    <NUMDOC>FC70                               </NUMDOC>
    <LIBECR>D�l�gation signature(s)                                     </LIBECR>
    <GAPROFIL>JFL       </GAPROFIL>
    <GADATE>20130524</GADATE>
    <GAHEURE>133716</GAHEURE>
    <FILLER />
  </W-TIBCPTR>
  <W-TIBCPTR>
    <TYPTRT>FDR2 R</TYPTRT>
    <CLAFCNO VALIDE="NON">5b8</CLAFCNO>
    <NUMCLI>0052830</NUMCLI>
    <TYPACT>E</TYPACT>
    <NATCTRT>GPE</NATCTRT>
    <CLDEVCDE>EUR</CLDEVCDE>
    <AFCACH>999</AFCACH>
    <NUMACH>0026627</NUMACH>
    <DEVACH>EUR</DEVACH>
    <NUMDOC>130512                             </NUMDOC>
    <LIBECR>D�l�gation signature(s)                                     </LIBECR>
    <GAPROFIL>JFL       </GAPROFIL>
    <GADATE>20130527</GADATE>
    <GAHEURE>095045</GAHEURE>
    <FILLER />
  </W-TIBCPTR>
</W-TIBCPTRs>

string csvSeparator = ",";
Func<string, string> escapeValue = val => val;

string xml = textBox1.Text;
XDocument doc = XDocument.Parse(xml);

var headers = doc.Root
           .Elements()
           .First()
           .Elements()
           .Select(el => el.Name.LocalName);

var headerRow = string.Join(csvSeparator, headers);

 var rows = from el in doc.Root.Elements()
           let values = from prop in el.Elements()
           select escapeValue(prop.Value)
           let row = string.Join(csvSeparator, values)
           select row;

IEnumerable<string> csvLines = new[] { headerRow }.Concat(rows);

Data at the root level is invalid. Line 1, position 1