SOAP service exception after second call

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;
}