Error message when using OpenERP-Jasper integration on our server with XML data source

I made a Jasperreports document, using an XML data source. It works fine on my pc, but when I install it on the server (Ubuntu), it doesn't work. When I look into the OpenERP log, I see the following messages appear:

2012-06-16 07:21:33,967 26535 INFO ? openerp.addons.jasper_reports.jasper_report: Requested report: '/home/openerp/openerp/6.1_bzr/server/openerp/addons/jasper_reports/custom_reports/OpportunitiesSmartSolutionXML.jrxml'
2012-06-16 07:21:33,970 26535 INFO ? openerp.addons.jasper_reports.jasper_report: Temporary data file: '/tmp/tmpt5rva8'
2012-06-16 07:21:33,991 26535 ERROR ? openerp.service.web_services: Exception: not well-formed (invalid token): line 1, column 9
Traceback (most recent call last):
  File "/home/openerp/openerp/6.1_bzr/server/openerp/service/web_services.py", line 727, in go
    (result, format) = obj.create(cr, uid, ids, datas, context)
  File "/home/openerp/openerp/6.1_bzr/server/openerp/addons/jasper_reports/jasper_report.py", line 257, in create
    return r.execute()
  File "/home/openerp/openerp/6.1_bzr/server/openerp/addons/jasper_reports/jasper_report.py", line 144, in execute
    pages = self.executeReport( dataFile, outputFile, subreportDataFiles )
  File "/home/openerp/openerp/6.1_bzr/server/openerp/addons/jasper_reports/jasper_report.py", line 223, in executeReport
    return server.execute( connectionParameters, self.reportPath, outputFile, parameters )
  File "/home/openerp/openerp/6.1_bzr/server/openerp/addons/jasper_reports/JasperReports/JasperServer.py", line 86, in execute
    return self.proxy.Report.execute( *args )
  File "/usr/lib/python2.7/xmlrpclib.py", line 1224, in __call__
    return self.__send(self.__name, args)
  File "/usr/lib/python2.7/xmlrpclib.py", line 1575, in __request
    verbose=self.__verbose
  File "/usr/lib/python2.7/xmlrpclib.py", line 1264, in request
    return self.single_request(host, handler, request_body, verbose)
  File "/usr/lib/python2.7/xmlrpclib.py", line 1297, in single_request
    return self.parse_response(response)
  File "/usr/lib/python2.7/xmlrpclib.py", line 1467, in parse_response
    p.feed(data)
  File "/usr/lib/python2.7/xmlrpclib.py", line 557, in feed
    self._parser.Parse(data, 0)
ExpatError: not well-formed (invalid token): line 1, column 9
2012-06-16 07:21:34,215 26535 ERROR ? openerp.netsvc: not well-formed (invalid token): line 1, column 9
(<class 'xml.parsers.expat.ExpatError'>, ExpatError('not well-formed (invalid token): line 1, column 9',), <traceback object at 0x7f42d6bad3b0>)
2012-06-16 07:21:34,216 26535 ERROR ? openerp.addons.web.common.http.HttpRequest.dispatch: An error occurred while handling a json request
Traceback (most recent call last):
  File "/home/openerp/openerp/6.1_bzr/server/openerp/addons/web/common/http.py", line 260, in dispatch
    r = method(controller, self, **self.params)
  File "/home/openerp/openerp/6.1_bzr/server/openerp/addons/web/controllers/main.py", line 1833, in index
    req.session._db, req.session._uid, req.session._password, report_id)
  File "/home/openerp/openerp/6.1_bzr/server/openerp/addons/web/common/openerplib/main.py", line 117, in proxy
    result = self.connector.send(self.service_name, method, *args)
  File "/home/openerp/openerp/6.1_bzr/server/openerp/addons/web/common/http.py", line 596, in send
    fault = xmlrpclib.Fault('warning -- ' + e.name + '\n\n' + e.value, '')
TypeError: coercing to Unicode: need string or buffer, tuple found

The error message talks about not well-formed (invalid token) on line 1, column 9. I have been looking into the temporary data file, and I have the following information on line 1 (line with field names), and line 2 (first data line - I only changed the name of the prospect):

Expected_Closing-date_deadline,Creation_Date-create_date,Name-name,Next_Action_Date-date_action,Next_Action-title_action,Stage_Name-name,Expected_Revenue-planned_revenue,Probability-probability,
2012-06-20 00:00:00,2012-05-25 09:03:25,ProspectName,2012-06-08 00:00:00,Follow up offerte,4-Negotiation,100000.0000000000,70.0000000000,

I hope that you can help me how to resolve my problem. Thanks.

Wim