If your payment are working well before and you started getting “Things do not appear to be working at the moment” error suddenly then it seems you made the API call right and  that why you moved to  PayPal for payment. So it means nothing wrong with your call. It is because of some internal issue at PayPal or some issue with your browser cache.

Try to clean your browser cache or try new browser window. It worked for me. Also try switch between LIVE and SandBox

If you are getting this payment during integration visit following link

https://www.paypal.com/us/smarthelp/article/ts2067