The content of Oslo merchant validation is incorrect

Hi.

I am integrating with the Apple Pay on my website. I am trying to perform a request via Postman to register a merchant - just testing if body of request is valid, if it actually registers a merchant etc. While I am doing so I get the following error:

{
    "statusMessage": "Payment Services Exception Domain check failed with the following errors. No domains were registered.\nDomain verification failed for pspId=ENCRYPTED_MERCHANT_ID domain=xyz.com url=/.well-known/apple-developer-merchantid-domain-association errorMessage=The content of Oslo merchant validation is incorrect",
    "statusCode": "400"
}

Note that domain name and pspId were censored for security reason.

I have no idea what does that error mean.

1. Our domain is verified. We have domain verification file on the endpoint /.well-known/apple-developer-merchantid-domain-association errorMessage - otherwise our domain could not be registered. I have also checked it manually many times if it is there and everything looked fine.
2. We have an active payment processing certificate.
3. We have a merchant identity certificate. I use it on the serwer and on Postman for both apple-pay-gateway.apple.com and apple-pay-gateway-cert.apple.com.
4. Requesta sent to Sandbox pass and return status 200.

Request body:

{
    "domainNames": ["xyz.com"],
    "encryptTo": "merchant.com.our.identifier",
    "partnerInternalMerchantIdentifier": "xyz25",
    "partnerMerchantName": "Some name"
}

Data is replaced with placeholders. I am 100% sure that fields encryptTo and domainNames are valid.

Thank you in advance for any help.