Sandbox Payment: Things don’t appear to be working right now.

[AndyVu]

I tried to test payment for my website using Paypal Sandbox, but seems not working and always get this message: "Things don’t appear to be working right now.".

I have checked all settings and make sure sandbox is enabled. I also tried to clear out all cache and cookies. Even open the website in incognito mode.

Could anyone help or tell me what's is going on?

Thanks

[AndyVu]

I got this message in the console 

Failed to load resource: the server responded with a status of 500 (Internal Server Error)

POST https://www.sandbox.paypal.com/webapps/hermes/api/batch/setbuyer 500 (Internal Server Error)

POST https://www.sandbox.paypal.com/webapps/hermes/api/checkout/6BH820117E963240N/session/create 500 (Internal Server Error)

pa.js:2 Uncaught TypeError: Cannot read property 'data' of undefined
at kt (pa.js:2)
at pa.js:2
at Array.forEach (<anonymous>)
at pt (pa.js:2)
at mt (pa.js:2)
at ft (pa.js:2)
at pa.js:2
at Array.forEach (<anonymous>)
at lt (pa.js:2)
at pa.js:2

[SpectrumBlast]

I also get the same error but when trying to add my MasterCard.

I usually wait for a few hours and it worked later, please fix this PayPal.