I am developing a .Net website to which i have integrated the salesforce using the WSDL file. I am getting an issue occasionally while trying the following.

1. I have an XML in the .net website where the SOQL queries resides. In the same XML file, i am having the Username, password and the security token of the SFDC org.
2. First up, i will read the login credentials from the XML file and login to the SFDC org.
3. After login, i will be fetching the SOQL query from the same XML file and trying to run the query in the salesforce to fetch a n number of rows.
4. While the query runs in the SFDC org, i am getting an error as below,

An unexpected error has occurred: There is an error in XML document (1, 1229).
   at System.Xml.Serialization.XmlSerializer.Deserialize(XmlReader xmlReader, String encodingStyle, XmlDeserializationEvents events)
   at System.Xml.Serialization.XmlSerializer.Deserialize(XmlReader xmlReader, String encodingStyle)
   at System.Web.Services.Protocols.SoapHttpClientProtocol.ReadResponse(SoapClientMessage message, WebResponse response, Stream responseStream, Boolean asyncCall)
   at System.Web.Services.Protocols.SoapHttpClientProtocol.Invoke(String methodName, Object[] parameters)
   at WebReferences.SForceService.SforceService.query(String queryString) in 'my file path'
   at RowDI.DataReader.fGetData() in 'my file path'

Another important thing is, this application will be accessed by n number of users at the same time. It will run simulteanously and parallely. so, there can be chances where multiple users to login at the same time and fetch the query from XML file and get the records. This issue is occurring once for every 5 runs i.e,. occasionally occurring issue.

so is there anything that we are missing to do in this scenario? any ideas/suggestions will be very helpful and grateful to us asap, as this application is already in production environment.

Thanks

abivenkat