SOAP service exception after second call - soap

Question

I created a SOAP service that fetches some data from a remote server.
After second call I always get an exception:
stackTrace: org.w3c.dom.DOMException: WRONG_DOCUMENT_ERR: A node is used in a different document than the one that created it.
Any ideas what could be wrong?
function getInstance () {
return LocalServiceRegistry.createService('SaveNewCustomerService', new SaveNewCustomerServiceObject());
}
function SaveNewCustomerServiceObject() {};
SaveNewCustomerServiceObject.prototype.initServiceClient = function() {
this.webReference = webreferences.CustomerWS;
return this.webReference.getDefaultService();
}
SaveNewCustomerServiceObject.prototype.createRequest = function(svc, params) {
return params;
}
SaveNewCustomerServiceObject.prototype.execute = function(svc, requestObject) {
var customerRequestObjTest = new webreferences.CustomerWS.SaveNewCustomer();
if (requestObject != null) {
setObj(); //reduced
}
var result;
try{
result = svc.serviceClient.saveNewCustomer(customerRequestObjTest);
var a =result;
}catch(ex){
var e = ex; //
if(e.faultString == "Import error"){
log.info("Import error" + e.faultDetail);
}else{
log.info(e.faultDetail);
}
}
return result;
}
SaveNewCustomerServiceObject.prototype.parseResponse = function(svc, responseObject) {
return responseObject;
}