We experienced a similar issue 500 error, it took forever to troubleshoot. For context, angular/ionic app. You may try checking that your app ID is correctly configured in ALL config files, as one of our tricky little issues was an app-ID misconfiguration in our app's HTTP client. Good luck!

similar error here: <SKPaymentQueue: 0x281c302b0>: Payment completed with error: Error Domain=ASDErrorDomain Code=500 "(null)" UserInfo={client-environment-type=Sandbox, storefront-country-code=USA, NSUnderlyingError=0x281077240 {Error Domain=AMSErrorDomain Code=301 "Invalid Status Code" UserInfo={NSLocalizedDescription=Invalid Status Code, AMSURL=https://sandbox.itunes.apple.com/WebObjects/MZBuy.woa/wa/inAppBuy?REDACTED, AMSStatusCode=500, NSLocalizedFailureReason=The response has an invalid status code}}}