[SalesForce] “Cannot parse empty string” error when exporting using dataloader

I'm trying to run Data Loader from the command line and export all the fields for the contact object. We're using a third party app which has modified this standard object. I've defined a mapping file containing all the fields that are exported when you use the Data Loader tool itself. When I run it I get this error:

<?xml version="1.0" encoding="UTF-8"?><env:Envelope
   xmlns:env="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsd="http://www.w3.org/2001/XMLSchema"
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
 <env:Header>
  <CallOptions xmlns="urn:partner.soap.sforce.com">
   <client>DataLoaderPartnerBatch/36.0</client>
   <defaultNamespace xsi:nil="true" />
   <returnFieldDataTypes>false</returnFieldDataTypes>
  </CallOptions>
  <SessionHeader xmlns="urn:partner.soap.sforce.com">
   <sessionId>00D58000000Y0QB!ARIAQGhDjJnFwJK2qMmzmmvZxcopWdufXRDvTyEBFbs5W0icnBY6uR.Piy7YgTZclqb68jGOpIlVkPQ6uUGwQduc4__L1T7r</sessionId>
  </SessionHeader>
 </env:Header>
 <env:Body>
  <m:describeSObject xmlns:m="urn:partner.soap.sforce.com" xmlns:sobj="urn:sobject.partner.soap.sforce.com">
   <m:sObjectType>causeview__Volunteer_Role__c</m:sObjectType>
  </m:describeSObject>
 </env:Body>
</env:Envelope>

2016-06-23 17:04:16,771 INFO  [contactExportProcess] process.ProcessRunner run (ProcessRunner.java:145) - Creating Map
2016-06-23 17:04:16,781 INFO  [contactExportProcess] action.OperationInfo instantiateAction (OperationInfo.java:95) - Instantiating action for operation: extract
2016-06-23 17:04:16,786 INFO  [contactExportProcess] controller.Controller executeAction (Controller.java:120) - executing operation: extract
2016-06-23 17:04:16,789 INFO  [contactExportProcess] action.AbstractAction execute (AbstractAction.java:120) - Loading: extract
2016-06-23 17:04:16,798 ERROR [contactExportProcess] action.AbstractAction handleException (AbstractAction.java:204) - Exception occured during loading
2016-06-23 14:29:22,978 ERROR [contactExportProcess] action.AbstractAction handleException (AbstractAction.java:204) - Exception occured during loading java.lang.IllegalArgumentException: Cannot parse empty string

Above that snippet of XML are similar ones where the m:SObjectType is Account and User. The third party app is called Causeview, causeview__Volunteer_Role__c is a custom object. What could be causing this problem?