Apple Pay 504 Payment Services Exception Request timed out

Hi!

I'm trying to get Apple Pay Payment session following by this instruction

https://developer.apple.com/documentation/apple_pay_on_the_web/apple_pay_js_api/requesting_an_apple_pay_payment_session

1. At the first step I'm getting validation URL using Javascript API. It seems everything is fine at this step because I can receive "validation URL".

2. Next I'm sending server side query to this "validation URL". But this query gets failed every time. It's always the same error: {

"statusMessage": "Payment Services Exception Request timed out",

"statusCode": "504"

}

This is the data I'm trying to send

{"merchantIdentifier":"merchant.com.my-store","displayName":"My Store","initiative":"web","initiativeContext":"my-store.com"}

I'm also using valid Merchant Identity certificate to sign this request. I know this because without a valid certificate I'm getting another type of error:

error:14094410:SSL routines:SSL3_READ_BYTES:sslv3 alert handshake failure

So it seems I have no problem with transport layer because I can receive a valid JSON as a response.

But please tell me why Apple server is always respond with 504 (Request timeout) error?

Am I doing something wrong?