Hey Roman, Did you ever get this resolved (either through someone @ Apple or through personal experience and server logs)? Thanks!

We are also experiencing this error when calling GET https://api.storekit-sandbox.itunes.apple.com/inApps/v1/subscriptions/{originalTransactionId}. Came to light during automated tests. But the error hasn't been consistent. We run tests twice a day, and sometimes they passed, sometimes they didn't. We've gotten this error code a few times from a few other users in production when I query. I guess it's just a general error code. But it is a recent change that's causing this because our tests were always working before.

@crauss77 We are also not getting the error anymore now, but we're getting the following response for a sandbox account that has a definite purchase history: [environment] => Sandbox [bundleId] => com.brainpop.brainpopfeaturedmovie [data] => Array ( ) Is it all working for you?