paypal sandbox - In-Context Checkout: No response handler found for post message response error -

i use advanced in-context checkout integration.

i ajax , utilize these functions, docs explain it:

paypal.checkout.setup() paypal.checkout.initxo() paypal.checkout.startflow() paypal.checkout.closeflow() 

and annoying error after execute payment step. didn't bother explain anywhere api docs. missing.

new error("no response handler found post message response "+t.name+" in "+window.location.href);

what error mean? kind of response handler post message response expected? how provide it?

my client side code simple following:

oncheckoutbtnclick(ev) {     paypal.checkout.initxo();     $.post('/checkout', {         user: json.stringify(this.props.user),         data: json.stringify(this.props.wcs),     })     .done(res => {         paypal.checkout.startflow(res.approval_url);     })     .fail(err => {         paypal.checkout.closeflow();     }); }, 

and server side follows:

router.post('/checkout', (req, res, next) => {     var payment_json = {          // huge config create payment         // pivotal value "return_url"         redirect_urls: {             "return_url": "http://example.com:3000/complete_payment",             "cancel_url": "http://example.com:3000/cancel_payment"         },     } }); 

and return_url code

router.get('/complete_payment', (req, res, next) => {     paypal.payment.execute(req.query.paymentid, execute_payment_json, function (err, payment) {          // after step error in thrown on client         res.redirect('http://example.com:3000/show_complete_page_to_buyer');     }) }) 

it seems sandbox problem only. see error time in sandbox , newer seen in production mode. have mailed paypal developer complaining on many difference between sandbox , production.