ListSoapClient UpdateListItems error

Hi everybody!

   First of all, thanks for reading...

   I have a WCF service that consume SharePoint Web Service to update list item. It gives me a Microsoft.SharePoint.SoapServer.SoapServerException:

Exception of type 'Microsoft.SharePoint.SoapServer.SoapServerException' was thrown. - System.Collections.ListDictionaryInternal - mscorlib - 
Server stack trace: 
   at System.ServiceModel.Channels.ServiceChannel.HandleReply(ProxyOperationRuntime operation, ProxyRpc& rpc)
   at System.ServiceModel.Channels.ServiceChannel.Call(String action, Boolean oneway, ProxyOperationRuntime operation, Object[] ins, Object[] outs, TimeSpan timeout)
   at System.ServiceModel.Channels.ServiceChannelProxy.InvokeService(IMethodCallMessage methodCall, ProxyOperationRuntime operation)
   at System.ServiceModel.Channels.ServiceChannelProxy.Invoke(IMessage message)

Here is my code:

ListsWS.ListsSoapClient lsc = new ListsWS.ListsSoapClient();
                lsc.ClientCredentials.Windows.AllowedImpersonationLevel = System.Security.Principal.TokenImpersonationLevel.Impersonation;
                lsc.ClientCredentials.Windows.ClientCredential = new NetworkCredential(user, pass, dom);

XElement list = lsc.GetListItems(guidList, null, query, null, null, null, null);

List<XElement> results = list.Descendants().ToList();

XElement updateItem = results[0];
updateItem .SetAttributeValue("ows_DeviceToken", "value to update");

lsc.UpdateListItems(guidList, updateItem);

I've also tried this code:

ListsWS.ListsSoapClient lsc = new ListsWS.ListsSoapClient();
                lsc.ClientCredentials.Windows.AllowedImpersonationLevel = System.Security.Principal.TokenImpersonationLevel.Impersonation;
                lsc.ClientCredentials.Windows.ClientCredential = new NetworkCredential(user, pass, dom);

XElement list = lsc.GetListItems(guidList, null, query, null, null, null, null);

List<XElement> results = list.Descendants().ToList();

XElement updateItem = results[0];
string strBatch = "<Method ID='1' Cmd='Update'>" +"<Field Name='ID'>1</Field>" +"<Field Name='DeviceToken'>Value to update</Field></Method>";

XElement elBatch = new XElement(strBatch);
elBatch.SetAttributeValue("OnError", "Continue");
lsc.UpdateListItems(list, elBatch);

But gives me this error:

Name cannot begin with the '<' character, hexadecimal value 0x3C. - System.Collections.ListDictionaryInternal - System.Xml -    at System.Xml.XmlConvert.VerifyNCName(String name, ExceptionType exceptionType)
   at System.Xml.Linq.XNamespace.GetName(String localName, Int32 index, Int32 count)
   at System.Xml.Linq.XName.Get(String expandedName)

Thank you very much for your help!